[AArch64] Don't merge sp decrement into later stores when using WinCFI

This matches the corresponding existing case in
AArch64LoadStoreOpt::findMatchingUpdateInsnForward.

Both cases could also be modified to check
MBBI->getFlag(FrameSetup/FrameDestroy) instead of forbidding any
optimization involving SP, but the effect is probably pretty much
the same.

Differential Revision: https://reviews.llvm.org/D88541