Fix register hinting for phis.

Fix register hinting for phis.

The way it was implemented did not always work for phis for live ranges extending past the loop, because the hint itself stayed uncoverted until after register allocation inside the loop was completed.

For the loop like this:

f() {
  var x = 1.0,  y = 2.0;
  for (var i = 0; i < 100; i++) x += y;
  return x;
}

That used to generate:

loop:
  cmp ecx,0xc8
  jnl ->exit
  mov xmm3, xmm2
  addsd xmm3, xmm1
  mov xmm2, xmm3
  add ecx,0x2
  jmp ->loop

With this change we generate:

loop:
  cmp ecx,0xc8
  jnl ->exit
  addsd xmm2,xmm1
  add ecx,0x2
  jmp ->loop

R=fschneider@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=40024

[Vyacheslav Egorov (Google), 2014-09-09 10:50:02]

PTAL

[Florian Schneider, 2014-09-09 10:53:40]

lgtm

[Vyacheslav Egorov (Google), 2014-09-09 11:27:07]

Committed patchset #1 (id:1) manually as 40024 (presubmit successful).