[GlobalISel] Fix non-pow-2 legalization of s56 stores.

s56 stores are broken down into s32 + s24 stores. During this step
both of those new stores use an anyextended s64 value, resulting in
truncating stores. With s56, the s24 requires another lower step to
make it legal, and we were crashing because we didn't expect non-pow-2
stores to also be truncating as well.

Differential Revision: https://reviews.llvm.org/D106183