[x86, BMI] add TLI hook for 'andn' and use it to simplify comparisons

For the sake of minimalism, this patch is x86 only, but I think that at least
PPC, ARM, AArch64, and Sparc probably want to do this too.

We might want to generalize the hook and pattern recognition for a target like
PPC that has a full assortment of negated logic ops (orc, nand).

Note that http://reviews.llvm.org/D18842 will cause this transform to trigger
more often.

For reference, this relates to:
https://llvm.org/bugs/show_bug.cgi?id=27105
https://llvm.org/bugs/show_bug.cgi?id=27202
https://llvm.org/bugs/show_bug.cgi?id=27203
https://llvm.org/bugs/show_bug.cgi?id=27328

Differential Revision: http://reviews.llvm.org/D19087

llvm-svn: 268858