[FastISel][ARM] Fix a think-o in my previous commit (r215682).

We actually need to return the register into which we materialized the constant
and not just "true" for success. This code is currently partially dead, that is
why it didn't trigger any failures yet. Once I change the order of the constant
materialization this code will be fully exercised.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215727 91177308-0d34-0410-b5e6-96231b3b80d8