[CodeGen] Add MVT::FIRST_VALUETYPE to avoid explicit 0. NFC.

[CodeGen] Add MVT::FIRST_VALUETYPE to avoid explicit 0. NFC.

Many places reference MVT::LAST_VALUETYPE when iterating over all
valid MVTs, but they usually start with 0.
With FIRST_VALUETYPE, we can avoid explicit constants when we really
should be using MVT::SimpleValueType.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225362 91177308-0d34-0410-b5e6-96231b3b80d8