MaglevCompilationInfo stores the overall compilation information (zone,
graph, labeller, etc.), while MaglevCompilationUnit stores per-function
information (function, bytecode analysis, register count, etc.).

Without inlining, these are 1:1 and we've been pretty sloppy in deciding
which to pass around. Once we implement inlining though, we want to be
careful to pass MaglevCompilationInfo where we're processing the whole
graph, and MaglevCompilationUnit where we're processing something
function-specific.

This does the pre-work of cleaning this up in preparation for inlining.

Bug: v8:7700
Change-Id: Ic50fdd97e56f6c963ab490bd419eb65fe0873688
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3596162
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80051}