[FastISel][ARM] Fix a bug in the integer materialization code.

getRegClassFor returns the incorrect register class when in Thumb2 mode.
This fix simply manually selects the register class as in the code just a few
lines above.

There is no test case for this code, because the code is currently
unreachable. This will be changed in a future commit and existing test
cases will exercise this code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215583 91177308-0d34-0410-b5e6-96231b3b80d8