Constant reloading of the null reference impacted some wasm-gc
benchmarks. This CL speeds some benchmarks by >5%.
Note: This solution is not ideal as it technically generates invalid
turbofan graphs. It is temporary until a proper optimization eliminating
excessive loads is implemented.

Change-Id: I7afa6fb8857f5dba3dde715bd30fe868ad06d92c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2704668
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72894}