When finishing a one-armed if, the else state has to be merged into the
end state. We did this before by switching to the else state, then
doing the merge. This CL changes this to avoid the switch.

Drive-by: Add a few missing "const" qualifiers. The style guide forbids
non-const l-value references.

R=titzer@chromium.org

Bug: v8:8423, v8:6600
Change-Id: Iab2aeca393147fba55493bebabd27bc4d77baa0f
Reviewed-on: https://chromium-review.googlesource.com/c/1375656Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58222}