[InstCombine] change select type to eliminate bitcasts

This solves a secondary problem seen in PR6137:
https://llvm.org/bugs/show_bug.cgi?id=6137#c6

This is similar to the bitwise logic op fold added with:
https://reviews.llvm.org/rL287707

And like that patch, I'm artificially restricting the
transform from vector <-> scalar types until we're sure
that the backend can handle that. 

llvm-svn: 288584