[AArch64][GlobalISel] Use wzr/xzr for 16 and 32 bit stores of zero

We weren't performing this optimization on 16 and 32 bit stores. SDAG happily
does this though.

e.g. https://godbolt.org/z/cWocKr

This saves about 0.2% in code size on CTMark at -O3.

Differential Revision: https://reviews.llvm.org/D84568