[mlir][func] Move return-type verification from ReturnOp to FuncOp (#184153)

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.