[mlir][spirv] Don't return value when cannot fold spirv.bitcast

[antiagainst]

Summary

Returing a value would make the canonicalization infrastructure
think that folding succeeded so the pattern will be tried again
when invoked via, e.g., applyPatternsAndFoldGreedily and
eventually fail due to not converging after 10 times by default.

mlir/lib/Dialect/SPIRV/IR/SPIRVCanonicalization.cpp

void spirv::AccessChainOp::getCanonicalizationPatterns(
   results.add<CombineChainedAccessChain>(context);
 }
 
 //===----------------------------------------------------------------------===//
 // spirv.BitcastOp
 //===----------------------------------------------------------------------===//
 
 OpFoldResult spirv::BitcastOp::fold(ArrayRef<Attribute> /*operands*/) {
-  Value arg = getOperand();
-  if (getType() == arg.getType())
-    return arg;
+  Value curInput = getOperand();
+  if (getType() == curInput.getType())
+    return curInput;
 
   // Look through nested bitcasts.
-  if (auto bitcast = arg.getDefiningOp<spirv::BitcastOp>()) {
-    Value nestedArg = bitcast.getOperand();
-    if (nestedArg.getType() == getType())
-      return nestedArg;
+  if (auto prevCast = curInput.getDefiningOp<spirv::BitcastOp>()) {
+    Value prevInput = prevCast.getOperand();
+    if (prevInput.getType() == getType())
+      return prevInput;
 
-    getOperandMutable().assign(nestedArg);
+    getOperandMutable().assign(prevInput);
     return getResult();
   }
 
   // TODO(kuhar): Consider constant-folding the operand attribute.
-  return getResult();
+  return {};
 }
 
 //===----------------------------------------------------------------------===//
 // spirv.CompositeExtractOp
 //===----------------------------------------------------------------------===//
 
 OpFoldResult spirv::CompositeExtractOp::fold(ArrayRef<Attribute> operands) {
   if (auto insertOp =