[compiler] Prevent extra copies of RpoNumber

This fixes the following compiler warning:

```
src/compiler/backend/mid-tier-register-allocator.cc:2046:26: warning: loop variable 'succ' of type 'const
      v8::internal::compiler::RpoNumber' creates a copy from type 'const v8::internal::compiler::RpoNumber'
      [-Wrange-loop-analysis]
    for (const RpoNumber succ : block->successors()) {
                         ^
src/compiler/backend/mid-tier-register-allocator.cc:2046:10: note: use reference type 'const
      v8::internal::compiler::RpoNumber &' to prevent copying
    for (const RpoNumber succ : block->successors()) {
         ^~~~~~~~~~~~~~~~~~~~~~
                         &
```

Signed-off-by: Darshan Sen <raisinten@gmail.com>
Change-Id: I41a9b6aa1ee16bf140f326410ae864b425509c2d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3538285


Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79616}