[InstCombine] change bitwise logic type to eliminate bitcasts

In PR27925:
https://llvm.org/bugs/show_bug.cgi?id=27925

...we proposed adding this fold to eliminate a bitcast. In D20774, there was 
some concern about changing the type of a bitwise op as well as creating 
bitcasts that might not be free for a target. However, if we're strictly 
eliminating an instruction (by limiting this to one-use ops), then we should 
be able to do this in InstCombine.

But we're cautiously restricting the transform for now to vector types to
avoid possible backend problems. A transform to make sure the logic op is
legal for the target should be added to reverse this transform and improve
codegen.

Differential Revision: https://reviews.llvm.org/D26641

llvm-svn: 287707