[llvm] Initialize and move UseUpRegs outside of `union` MemOp struct.

Asan complained about uninitialized bool

`invalid-bool-load`
llvm/lib/Target/X86/AsmParser/X86Operand.h:389:12: runtime error: load
of value 171, which is not a valid value for type 'bool'

Differential Revision: https://reviews.llvm.org/D122405