[FastISel][ARM] Use MOVT/MOVW if the subtarget requests it.

This change is also in preparation for a future change to make sure that
the constant materialization uses MOVT/MOVW when available and not a load
from the constant pool.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215584 91177308-0d34-0410-b5e6-96231b3b80d8