[AArch64] Improve access to fixed-width object when stack has SVE.

When the stack has SVE objects, fixed-width objects are often better accessed
from the SP, instead of the FP, because part/all of the fixed-width offset
can be folded into the (non-scalable) addressing mode, where otherwise an
ADDVL would be required.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D120738