[x86] Faster/shorter code for stack checks.

[x86] Faster/shorter code for stack checks.

Avoid loading the stack pointer and the stack limit into allocatable
registers first, but generate a cmp rsp,[limit] instead.

R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/989203002

Cr-Commit-Position: refs/heads/master@{#27065}