Account for the constant pool pointer slot during register allocation
data initialization.

R=danno@chromium.org, titzer@chromium.org, bmeurer@chromium.org, mcilroy@chromium.org,
TEST=cctest/test-run-machops/RunSpillConstantsAndParameters
BUG=

Review URL: https://codereview.chromium.org/1317123003

Cr-Commit-Position: refs/heads/master@{#30430}