The idea behind this change is to restore some information about
pre-ssa values to aid register allocation in sharing spill slots and
reusing registers for connected live ranges.

By itself, this change does not improve much but it allows upcoming
changes to freely spill and reload ranges without worrying about
keeping the assignment stable.

Change-Id: I9320522592546655cc8fd0236d45fe075276a49e
Reviewed-on: https://chromium-review.googlesource.com/c/1375665
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58688}