[InstCombine] invert canonicalization for cast of signbit test

The existing transform was wrong in 3 ways:
1. It created an extra instruction when the source and dest types don't match.
2. It did not account for an extra use of the icmp, so could create 2 extra insts.
3. It favored bit hacks over icmp (icmp generally has better analysis).

This fixes #54692 (modeled by the PhaseOrdering tests).

This is a minimal step to fix the bug, but we should likely invert
the sibling transform for the "is negative" pattern too.

The backend should be able to invert this back to a shift if that
leads to better codegen.