The number of constants stored in locals and the merge region can be
arbitrarily big, thus generating arbitrarily long code for a single
`br_if`. This happened in particular for unoptimized code.

This CL solves this by materializing all constants (in registers or on
the stack) before doing a conditional branch. This ensures that in a
series of `br_if`s, each constant is only spilled once instead of on
each single branch.

For the linked bug, this reduces the total generated code size by ~36%.

R=thibaudm@chromium.org

Bug: chromium:1117033
Change-Id: I84ea2ea9ba4d3de9b042ceb223af15c3d73dc5b8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2364498Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69485}