[InstCombine] morph an existing instruction instead of creating a new one

[InstCombine] morph an existing instruction instead of creating a new one

One potential way to make InstCombine (very slightly?) faster is to recycle instructions
when possible instead of creating new ones. It's not explicitly stated AFAIK, but we don't
consider this an "InstSimplify". We could, however, make a new layer to house transforms
like this if that makes InstCombine more manageable (just throwing out an idea; not sure
how much opportunity is actually here).

Differential Revision: https://reviews.llvm.org/D31863

llvm-svn: 300067