Before computing the merge state, compute the list of free registers.
This list is used when we need a register for ensuring single register
use. It avoids picking a register which is in use by a not-yet-visited
merge slot, which would then have to be moved to another register or to
the stack.

On epic, this saves ~10.5% generated code size in Liftoff.

R=herhut@chromium.org

Bug: v8:8423
Change-Id: If9fac24abbfe2273bf18d9cb287c7c05a59b2118
Reviewed-on: https://chromium-review.googlesource.com/c/1387488
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Stephan Herhut <herhut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58435}