[InstCombine] Remove code duplication between InstCombiner.h and InstCombineInternal.h.

The class in InstCombineInternal.h inherits from InstCombiner.h.
I think this split was created when target specific InstCombines
were moved to go through TTI.

I had to update some of the code in InstCombiner.h to match changes
that had been made to InstCombineInternal.h.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D140230