[CGP] eliminate a sub instruction in memcmp expansion

As noted in D34071, there are some IR optimization opportunities that could be 
handled by normal IR passes if this expansion wasn't happening so late in CGP.

Regardless of that, it seems wasteful to knowingly produce suboptimal IR here, 
so I'm proposing this change:
  %s = sub i32 %x, %y
  %r = icmp ne %s, 0
    =>
  %r = icmp ne %x, %y

Changing the predicate to 'eq' mimics what InstCombine would do, so that's just
an efficiency improvement if we decide this expansion should happen sooner.

The fact that the PowerPC backend doesn't eliminate the 'subf.' might be 
something for PPC folks to investigate separately.

Differential Revision: https://reviews.llvm.org/D34416

llvm-svn: 306471