[AArch64][GlobalISel] Improve legalization for odd-sized G_ICMP/G_CONSTANT

We were handing types like s88 like

1) clamp to the range
2) widen to the next power of 2

This isn't desirable because it causes an odd breakdown for types like s88.
If we widen to the next power of 2 (s128) first, then we get a clean breakdown
when we clamp back to s64.

Differential Revision: https://reviews.llvm.org/D106998