[Stackmaps] Move the target-independent frame index elimination for stackmaps and patchpoints into target-specific code.

The lowering of the frame index for stackmaps and patchpoints requires some
target-specific magic and should therefore be handled in the target-specific
eliminateFrameIndex method.

This is related to <rdar://problem/16106219>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201904 91177308-0d34-0410-b5e6-96231b3b80d8