Move the operand count and type checks for func.return from
ReturnOp::verify() into a new FuncOp::verify(). The verifier iterates
all blocks in the callable region, skipping terminators that are not
func.return (e.g. llvm.return or test.return that may appear during
dialect conversion).
Fix several invalid-IR tests that had func.func return types
inconsistent with the actual func.return operands. Previously these
mismatches were silent because block verification stopped at an earlier
expected error before reaching the func.return; now that
FuncOp::verify() runs before body verification, the return types must be
consistent.
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