we may introduce moves that are redundant in the context of
moves on subsequent instructions. Currently, we only detect such
redundancies by allowing moves to skip over Nop instructions (true
nops, with no input/output). We can also skip over other cases, for
example over constant definitions (nop with an output), since whatever
moves happen above it do not influence the instruction's outcome.

We may be able to handle other cases, too - in subsequent CLs.

BUG=

Review URL: https://codereview.chromium.org/1422333003

Cr-Commit-Position: refs/heads/master@{#31662}