Increase limit for OpActions array

The OpActions array had a limit of 32 value types, so change it to use
MVT::MAX_ALLOWED_VALUETYPE in its declaration and change the accesses to
this array to work with a VT.getSimpleVT() that is larger than 32.

Also, add a comment to the place where MVT::MAX_ALLOWED_VALUETYPE is
defined indicating that it must be a multiple of 32.

This is part of the work allow MVT::LAST_VALUETYPE be greater than 32.

llvm-svn: 74130