[CodeGen] Prefer static frame index for STATEPOINT liveness args

Summary:
If a given liveness arg of STATEPOINT is at a fixed frame index
(e.g. a function argument passed on stack), prefer to use this
fixed location even the address is also in a register. If we use
the register it will generate a spill, which is not necessary
since the fixed frame index can be directly recorded in the stack
map.

Patch by Cherry Zhang <cherryyz@google.com>.

Reviewers: thanm, niravd, reames

Reviewed By: reames

Subscribers: cherryyz, reames, anna, arphaman, llvm-commits

Differential Revision: https://reviews.llvm.org/D53889

llvm-svn: 347998