[AArch64][GlobalISel] Perform load/store extended reg folding with optsize

GlobalISel was only doing this with minsize. SDAG does this with optsize.

(See: `SelectionDAG::shouldOptForSize()`)

This is a 0.3% code size improvement for CTMark at -Os.

(Best: 1.1% improvements on lencod + pairlocalalign)

Differential Revision: https://reviews.llvm.org/D96451