The allocator for determining the location (reg/stack) for parameters
and return values can be constexpr. This avoids lazy initialization,
saving code size and execution time, and simplifying the implementation
significantly.

R=ahaas@chromium.org
CC=titzer@chromium.org

Change-Id: I295623cb1dad0f1537f7292dcf044f3d509588bb
Reviewed-on: https://chromium-review.googlesource.com/635163Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47602}