[FIRRTL] LowerToHW: guard against folded operations

In LowerToHW we copy the name from FIRRTL ops to the lowered HW ops.  We
often call `createOrFold` when creating the HW ops, which can mean that
there is no operation to copy the name to.  We were not properly
detecting this scenario, which could cause us to try to copy the name to
a null operation.  Most ops were already covered under a guard in a
generic code path, but div-like and subaccess-like operations needed to
be guarded.  Some of these problem are impossible to test right now as
we require folders to be implemented for some operations.

lib/Conversion/FIRRTLToHW/LowerToHW.cpp

@@ -2729,7 +2729,8 @@ FailureOr<Value> FIRRTLLowering::lowerSubindex(SubindexOp op, Value input) {
     result = builder.createOrFold<sv::ArrayIndexInOutOp>(input, iIdx);
   else
     result = builder.createOrFold<hw::ArrayGetOp>(input, iIdx);
-  tryCopyName(result.getDefiningOp(), op);
+  if (auto *definingOp = result.getDefiningOp())
+    tryCopyName(definingOp, op);
   return result;
 }
 
@@ -2752,7 +2753,8 @@ FailureOr<Value> FIRRTLLowering::lowerSubaccess(SubaccessOp op, Value input) {
     result = builder.createOrFold<sv::ArrayIndexInOutOp>(input, valueIdx);
   else
     result = createArrayIndexing(input, valueIdx);
-  tryCopyName(result.getDefiningOp(), op);
+  if (auto definingOp = result.getDefiningOp())
+    tryCopyName(definingOp, op);
   return result;
 }
 
@@ -2772,7 +2774,8 @@ FailureOr<Value> FIRRTLLowering::lowerSubfield(SubfieldOp op, Value input) {
     result = builder.createOrFold<sv::StructFieldInOutOp>(input, field);
   else
     result = builder.createOrFold<hw::StructExtractOp>(input, field);
-  tryCopyName(result.getDefiningOp(), op);
+  if (auto *definingOp = result.getDefiningOp())
+    tryCopyName(definingOp, op);
   return result;
 }
 
@@ -3635,7 +3638,8 @@ LogicalResult FIRRTLLowering::lowerDivLikeOp(Operation *op) {
   else
     result = builder.createOrFold<UnsignedOp>(lhs, rhs, true);
 
-  tryCopyName(result.getDefiningOp(), op);
+  if (auto *definingOp = result.getDefiningOp())
+    tryCopyName(definingOp, op);
 
   if (resultType == opType)
     return setLowering(op->getResult(0), result);

test/Conversion/FIRRTLToHW/lower-to-hw.mlir

@@ -245,7 +245,11 @@ firrtl.circuit "Simple"   attributes {annotations = [{class =
     // CHECK-NEXT: = comb.extract [[SHIFT]] from 0 : (i14) -> i4
     %31 = firrtl.dshr %25, %18 : (!firrtl.sint<4>, !firrtl.uint<14>) -> !firrtl.sint<4>
 
-    // CHECK-NEXT: comb.icmp bin ule {{.*}}, {{.*}} : i4
+    // Noop.
+    %c0_ui1 = firrtl.constant 0 : !firrtl.const.uint<1>
+    %32 = firrtl.dshr %in1, %c0_ui1 { name = "test" } : (!firrtl.uint<4>, !firrtl.const.uint<1>) -> !firrtl.uint<4>
+
+    // CHECK: comb.icmp bin ule {{.*}}, {{.*}} : i4
     %41 = firrtl.leq %in1, %4 : (!firrtl.uint<4>, !firrtl.uint<4>) -> !firrtl.uint<1>
     // CHECK-NEXT: comb.icmp bin ult {{.*}}, {{.*}} : i4
     %42 = firrtl.lt %in1, %4 : (!firrtl.uint<4>, !firrtl.uint<4>) -> !firrtl.uint<1>