[ARM] Optimize expressions like "return x != 0;" for Thumb1.

There's an existing optimization for x != C, but somehow it was missing
a special case for 0.

While I'm here, also cleaned up the code/comments a bit: the second
value produced by the MERGE_VALUES was actually dead, since a CMOV only
produces one result.

Differential Revision: https://reviews.llvm.org/D59616

llvm-svn: 357437