[AArch64][GlobalISel] Fold away lowered vector sign-extend of vector compares.

This fixes a long standing cause of awful code generation when legalization creates
G_SEXT(G_FCMP(...)), for example due to promoting the condition of a vector G_SELECT.

Since on AArch64 vector compares sign-extend the condition value, there's no need
for this extra G_SEXT. Unfortunately by the time we get to post-legalization these
G_SEXTs have already been lowered into shifts, so this combine is a bit more
involved than I'd ideally like. Oh well.

Differential Revision: https://reviews.llvm.org/D135078