[IRBuilder] Migrate most casts to folding API

Migrate creation of most casts to use the FoldXYZ rather than
CreateXYZ style APIs. This means that InstSimplifyFolder now
works for these, which is what accounts for the AMDGPU test changes.

GitOrigin-RevId: 4251aa7a6f78ab063cad6c2e0aeadd76a660efba
8 files changed