Fixes a problem in instruction selection with testing whether or not the

transformation:

(X op C1) ^ C2 --> (X op C1) & ~C2 iff (C1&C2) == C2

should be done.  

This change has been tested:
 Using a debug+asserts build:
   on the specific test case that brought this bug to light
   make check-all
   lnt nt
   using this clang to build a release version of clang
 Using the release+asserts clang-with-clang build:
   on the specific test case that brought this bug to light
   make check-all
   lnt nt

Checking in because Evan wants it checked in.  Test case forthcoming after
scrubbing.

llvm-svn: 154955