[GlobalISel] Require explicit G_BITCAST for same-size scalar G_MERGE_VALUES type changes

Generic MIR currently allows G_MERGE_VALUES to reassemble into a same-size scalar of a different type flavor, for example rebuilding f32 from pieces unmerged from i32. This is relied on in lowering and forced a workaround in LegalizationArtifactCombiner.h to emit G_BITCAST during artifact combining.

Tighten this so same-size scalar type changes require an explicit G_BITCAST instead of being represented directly by G_MERGE_VALUES. That should let us remove the current combiner patch.

Please see for more details: https://github.com/llvm/llvm-project/pull/200946