[CodeGen] Use MVT iterator_ranges in legality loops. NFC intended.

A few loops do trickier things than just iterating on an MVT subset,
so I'll leave them be for now.
Follow-up of r225387.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225392 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed