[X86] EmitCmp - always use cmpw with foldable loads (#92251)

By default, EmitCmp avoids cmpw with i16 immediates due to 66/67h length-changing prefixes causing stalls, instead extending the value to i32 and using a cmpl with a i32 immediate, unless it has the TuningFastImm16 flag or we're building for optsize/minsize.

However, if we're loading the value for comparison, the performance costs of the decode stalls are likely to be exceeded by the impact of the load latency of the folded load, the shorter encoding and not needing an extra register to store the ext-load.

This matches the behaviour of gcc and msvc.

Fixes #90355