AMDGPU: Partially revert Make WQM aware of Waterfall intrinsics

Reverting the extra check in the waterfall code that tries to determine if an
exec mask manipulation is being done inside the waterfall group.

Change-Id: If69a6a4d81c1519c18ce0ce082cef79723b8eb08

llvm/lib/Target/AMDGPU/SIInsertWaterfall.cpp

@@ -287,12 +287,6 @@ class SIInsertWaterfall : public MachineFunctionPass {
         return;
       }
 
-      auto Dst = TII->getNamedOperand(*Cand, AMDGPU::OpName::dst)->getReg();
-      if (Dst == AMDGPU::EXEC || Dst == AMDGPU::EXEC_LO) {
-        llvm_unreachable(
-            "EXEC manipulation within SI_WATERFALL_* group");
-      }
-
       // Iterate over the LastUseList to determine if this instruction has a
       // later use of a tagged last_use
       for (auto Use : LastUseList) {