Use a pointer type for target frame indices during statepoint lowering

Summary:
The type of the target frame index is intptr, not the type of the value we're
going to store into it.  Without this change we crash in the attached test case
when trying to type-legalize a TargetFrameIndex.

Patchpoint lowering types the target frame index as intptr as well.

Reviewers: reames, bogner, arsenm

Subscribers: arsenm, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D32256

llvm-svn: 301566