[AArch64][GlobalISel] Widen G_PHI before clamping it during legalization

This allows us to handle weird types like s88; we first widen to s128, then
clamp back down to s64.

https://godbolt.org/z/9xqbP46Mz

Also this makes it possible for GISel to legalize the case in pr48188.ll. It
now does the same thing as SDAG, although regalloc chooses different registers.

Differential Revision: https://reviews.llvm.org/D107417