[CGP] use subtract or subtract-of-cmps for result of memcmp expansion

As noted in the code comment, transforming this in the other direction might require 
a separate transform here in CGP given the block-at-a-time DAG constraint.

Besides that theoretical motivation, there are 2 practical motivations for the 
subtract-of-cmps form:

1. The codegen for both x86 and PPC is better for this IR (though PPC could be better still). 
   There is discussion about canonicalizing IR to the select form 
   ( http://lists.llvm.org/pipermail/llvm-dev/2017-July/114885.html ), 
   so we probably need to add DAG transforms for those patterns anyway, but this improves the 
   memcmp output without waiting for that step.

2. If we allow vector-sized chunks for the load and compare, x86 is better prepared to convert
   that to optimal code when using subtract-of-cmps, so another prerequisite patch is avoided
   if we choose to enable that.

Differential Revision: https://reviews.llvm.org/D34904

llvm-svn: 309597