[InstCombine][x86] try harder to convert blendv intrinsic to generic IR (PR38814)

Missing optimizations with blendv are shown in:
https://bugs.llvm.org/show_bug.cgi?id=38814

If this works, it's an easier and more powerful solution than adding pattern matching 
for a few special cases in the backend. The potential danger with this transform in IR
is that the condition value can get separated from the select, and the backend might 
not be able to make a blendv out of it again. I don't think that's too likely, but 
I've kept this patch minimal with a 'TODO', so we can test that theory in the wild 
before expanding the transform.

Differential Revision: https://reviews.llvm.org/D52059

llvm-svn: 342324