[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.

llvm-svn: 215583