[turbofan] Address minor TODOs in instruction selector.
[turbofan] Address minor TODOs in instruction selector.

R=dcarney@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1029383002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#27410}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh