A64 loads and stores can have much larger positive than negative
immediate offsets, and since most frame slots are below fp, we can
significantly improve accesses by basing them on sp instead. Typical
example:

    Before                  After
    mov x16, #-416
    str x20, [fp, x16]      str x20, [jssp, #32]

Notable benchmark results include lua_binarytrees, which improves by
about 7.5% on A57 and 5% on A53. Several other asm.js benchmarks gain
2-4%.

Review URL: https://codereview.chromium.org/1376173003

Cr-Commit-Position: refs/heads/master@{#32111}