[AArch64] Preserve register flags when promoting a load from store.

Summary:
This patch updates promoteLoadFromStore to use the store MachineOperand as the
source operand of the of the new instruction instead of creating a new
register MachineOperand. This way, the existing register flags are
preserved. 

This fixes PR33468 (https://bugs.llvm.org/show_bug.cgi?id=33468). 


Reviewers: MatzeB, t.p.northover, junbuml

Reviewed By: MatzeB

Subscribers: aemerson, rengolin, javed.absar, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D34402

llvm-svn: 305885