sufficient physical registers for all the virtual registers.

This should come in handy for frame elision. There, I noticed that even
for very small functions (==very few virtual registers), because we spill
the context/function marker ranges, we lose the frame elision
opportunity.

There is a subsequent change needed to fully help frame elision -
decoupling spilling of context & function marker from prologue.

BUG=

Review URL: https://codereview.chromium.org/1469743002

Cr-Commit-Position: refs/heads/master@{#32221}