[PM] Rename InstCombine.h to InstCombineInternal.h in preparation for

[PM] Rename InstCombine.h to InstCombineInternal.h in preparation for
creating a non-internal header file for the InstCombine pass.

I thought about calling this InstCombiner.h or in some way more clearly
associating it with the InstCombiner clas that it is primarily defining,
but there are several other utility interfaces defined within this for
InstCombine. If, in the course of refactoring, those end up moving
elsewhere or going away, it might make more sense to make this the
combiner's header alone.

Naturally, this is a bikeshed to a certain degree, so feel free to lobby
for a different shade of paint if this name just doesn't suit you.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226783 91177308-0d34-0410-b5e6-96231b3b80d8