- allows the optimization of emitted gap move code since the representation of the value in the register is known
- necessary preparation for vector register allocation
- prepare for slot sharing for any value of the same byte width

TBR=jarin@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1111323003

Cr-Commit-Position: refs/heads/master@{#28140}