Address feedback in https://reviews.llvm.org/D133637

https://reviews.llvm.org/D133637 fixes the problem where we should hash raw content of
register mask instead of the pointer to it.

Fix the same issue in `llvm::hash_value()`.

Remove the added API `MachineOperand::getRegMaskSize()` to avoid potential confusion.

Add an assert to emphasize that we probably should hash a machine operand iff it has
associated machine function, but keep the fallback logic in the original change.

Reviewed By: MatzeB

Differential Revision: https://reviews.llvm.org/D133747