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

Follow-up to rL342324 (D52059):

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

This is 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.

llvm-svn: 342806