We currently hold the register list for {move_src_regs_} in the
{StackTransferRecipe} class, and only compute src use counts for
executing the moves. Since the use counts and the register list are
redundant, just compute the use counts right away, and get rid of the
register list.
This saves one iteration of the register list in {ExecuteMoves}.

R=ahaas@chromium.org

Bug: v8:6600, v8:8423
Change-Id: I832fb0d1c1d3afe536289162a81a49b73313e7f4
Reviewed-on: https://chromium-review.googlesource.com/c/1397670
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58611}