[BACKEND] Fix heap-use-after-free in createAsyncCopy (triton-lang#3365)

There are two tests that failed under AddressSanitizer:
* test/TritonGPU/loop-pipeline.mlir
* python/test/regression/test_functional_regressions.py

with an error: 

```
==8475==ERROR: AddressSanitizer: heap-use-after-free on address 0x50c000bd0be0 at pc 0x557b03278847 bp 0x7ffd69b2c4a0 sp 0x7ffd69b2c498
READ of size 8 at 0x50c000bd0be0 thread T0
    #0 0x557b03278846 in getNextOperandUsingThisValue [third_party/llvm/llvm-project/mlir/include/mlir/IR/UseDefLists.h:43](https://cs.corp.google.com/piper///depot/google3/third_party/llvm/llvm-project/mlir/include/mlir/IR/UseDefLists.h?l=43&ws=aliia/3018&snapshot=215):58
    triton-lang#1 0x557b03278846 in operator++ [third_party/llvm/llvm-project/mlir/include/mlir/IR/UseDefLists.h:322](https://cs.corp.google.com/piper///depot/google3/third_party/llvm/llvm-project/mlir/include/mlir/IR/UseDefLists.h?l=322&ws=aliia/3018&snapshot=215):39
    triton-lang#2 0x557b03278846 in mlir::ResultRange::UseIterator::operator++() [third_party/llvm/llvm-project/mlir/lib/IR/OperationSupport.cpp:614](https://cs.corp.google.com/piper///depot/google3/third_party/llvm/llvm-project/mlir/lib/IR/OperationSupport.cpp?l=614&ws=aliia/3018&snapshot=215):5
    triton-lang#3 0x557affde38c4 in operator++ [third_party/llvm/llvm-project/llvm/include/llvm/ADT/iterator.h:281](https://cs.corp.google.com/piper///depot/google3/third_party/llvm/llvm-project/llvm/include/llvm/ADT/iterator.h?l=281&ws=aliia/3018&snapshot=215):5
    triton-lang#4 0x557affde38c4 in createAsyncCopy [third_party/triton/lib/Dialect/TritonGPU/Transforms/Pipeliner/MatmulLoopPipeline.cpp:117](https://cs.corp.google.com/piper///depot/google3/third_party/triton/lib/Dialect/TritonGPU/Transforms/Pipeliner/MatmulLoopPipeline.cpp?l=117&ws=aliia/3018&snapshot=215):26
    triton-lang#5 0x557affde38c4 in createAsyncLoad [third_party/triton/lib/Dialect/TritonGPU/Transforms/Pipeliner/MatmulLoopPipeline.cpp:135](https://cs.corp.google.com/piper///depot/google3/third_party/triton/lib/Dialect/TritonGPU/Transforms/Pipeliner/MatmulLoopPipeline.cpp?l=135&ws=aliia/3018&snapshot=215):3
    triton-lang#6 0x557affde38c4 in createAsynOps [third_party/triton/lib/Dialect/TritonGPU/Transforms/Pipeliner/MatmulLoopPipeline.cpp:501](https://cs.corp.google.com/piper///depot/google3/third_party/triton/lib/Dialect/TritonGPU/Transforms/Pipeliner/MatmulLoopPipeline.cpp?l=501&ws=aliia/3018&snapshot=215):5
    triton-lang#7 0x557affde38c4 in mlir::triton::preProcessLoopAndGetSchedule(mlir::scf::ForOp&, int, mlir::triton::PipeliningOption&) [third_party/triton/lib/Dialect/TritonGPU/Transforms/Pipeliner/MatmulLoopPipeline.cpp:740](https://cs.corp.google.com/piper///depot/google3/third_party/triton/lib/Dialect/TritonGPU/Transforms/Pipeliner/MatmulLoopPipeline.cpp?l=740&ws=aliia/3018&snapshot=215):7
    triton-lang#8 0x557affe01c0c in pipelineLoop [third_party/triton/lib/Dialect/TritonGPU/Transforms/Pipeliner/SoftwarePipeliner.cpp:76](https://cs.corp.google.com/piper///depot/google3/third_party/triton/lib/Dialect/TritonGPU/Transforms/Pipeliner/SoftwarePipeliner.cpp?l=76&ws=aliia/3018&snapshot=215):19
...
```
This is likely happening due to iterator being invalidated after
`alloc.erase()`.
This PR moves erases of allocations outside of a loop and fixes
heap-use-after-free issue.

Do you know if there is an easy way to run the tests under sanitizers
upstream? It would be handy if we can automate it, so we catch this kind
of errors early on.

lib/Dialect/TritonGPU/Transforms/Pipeliner/MatmulLoopPipeline.cpp

@@ -115,12 +115,16 @@ createAsyncCopy(scf::ForOp &forOp, tt::LoadOp loadOp, Value alloc,
     alloc.replaceAllUsesWith(viewLoad.getResult());
     alloc.erase();
   } else {
+    SmallVector<ttg::LocalAllocOp> allocsToErase;
     for (Operation *user : loadOp->getUsers()) {
       if (auto alloc = dyn_cast<ttg::LocalAllocOp>(user)) {
         alloc.replaceAllUsesWith(viewLoad.getResult());
-        alloc.erase();
+        allocsToErase.push_back(alloc);
       }
     }
+    for (auto alloc : allocsToErase) {
+      alloc.erase();
+    }
     auto sharedLoad =
         builder.create<ttg::LocalLoadOp>(loc, loadOp.getType(), viewLoad);
     loadOp->replaceAllUsesWith(sharedLoad->getResults());