When encountering a LoadStackPointer input to a comparison, generate a register
LocationOperand that points to the stack pointer. This can avoid unnecessary
spilling of the stack pointer.

Change-Id: Ifd1a5aaf22c9c594e653cf4689ba46587811c4d0
Reviewed-on: https://chromium-review.googlesource.com/1055568Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53161}