[IR][AutoUpgrade] Merge x86 mask load intrinsic upgrades. NFC.

Helps appease MSVC which is complaining about "fatal error C1061: compiler limit: blocks nested too deeply" - we already do the same thing for avx512.mask.store intrinsics.

This is only a stopgap solution until another else-if case needs adding - we really need to refactor this chain of ifs properly.