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Revert "RegAllocFast: Record internal state based on register units"

Authored by hans on Tue, Sep 15, 1:47 AM.

Description

Revert "RegAllocFast: Record internal state based on register units"

This seems to have caused incorrect register allocation in some cases,
breaking tests in the Zig standard library (PR47278).

As discussed on the bug, revert back to green for now.

Record internal state based on register units. This is often more
efficient as there are typically fewer register units to update
compared to iterating over all the aliases of a register.

Original patch by Matthias Braun, but I've been rebasing and fixing it
for almost 2 years and fixed a few bugs causing intermediate failures
to make this patch independent of the changes in
https://reviews.llvm.org/D52010.

This reverts commit 66251f7e1de79a7c1620659b7f58352b8c8e892e, and
follow-ups 931a68f26b9a3de853807ffad7b2cd0a2dd30922
and 0671a4c5087d40450603d9d26cf239f1a8b1367e. It also adjust some
test expectations.