This addresses an issue where the at-start splitting used
in the splintering mechanism was in conflict with the mechanics
used in linear allocator, in particular in the initial split/spill of
ranges for memory operands. We are already doing a split-at-start in
Greedy, so this change centralizes that to the base RegisterAllocator.

Verified locally that v8:4508 is addressed by this. Also, this fixes
the failures that required the revert
5308a999. See trybots at
issue 1425533002.

R=bmeurer@chromium.org,jarin@chromium.org
BUG= v8:4508
LOG=n

Review URL: https://codereview.chromium.org/1426583002

Cr-Commit-Position: refs/heads/master@{#31544}