[mlir][arith] Assert preconditions in BitcastOp::fold. (llvm#100743)

This PR adds an assertion to `BitcastOp::fold` that fails if that
function is called on invalid IR. That can happen when patterns, passes,
etc. create (invalid) IR using builders and folding is triggered on that
IR before verification, for example, through `OpBuilder::createOrFold`.
The new assert triggers earlier than previously in order to help getting
to the root cause faster.

Signed-off-by: Ingo Müller <ingomueller@google.com>

mlir/lib/Dialect/Arith/IR/ArithOps.cpp

@@ -1739,6 +1739,8 @@ OpFoldResult arith::BitcastOp::fold(FoldAdaptor adaptor) {
   APInt bits = llvm::isa<FloatAttr>(operand)
                    ? llvm::cast<FloatAttr>(operand).getValue().bitcastToAPInt()
                    : llvm::cast<IntegerAttr>(operand).getValue();
+  assert(resType.getIntOrFloatBitWidth() == bits.getBitWidth() &&
+         "trying to fold on broken IR: operands have incompatible types");
 
   if (auto resFloatType = llvm::dyn_cast<FloatType>(resType))
     return FloatAttr::get(resType,