This is a rudimentary introduction of hints. Primarily this helps with
allocating on the same register variables are defined (from instructions)
For dealing with phis, we need to introduce groups, in a subsequent
CL.

From the last CL (memory ops heuristics), this CL improves some
benchmarks - notably Life (11.94%) in Emscripten x64, and Memops
(Emscripten), 24% on x86; notable regressions: Memops in
AreWeFastYet (-14%, x64) and Corrections -25% on x86.

BUG=

Review URL: https://codereview.chromium.org/1329493004

Cr-Commit-Position: refs/heads/master@{#30534}