Revert of [Turbofan] Clean up register allocator and verifier code. (patchset #1 id:1 of https://codereview.chromium.org/2081443002/ )

Reason for revert:
[Sheriff] Speculative revert. Looks like this lets a few ignition+turbofan tests time out under tsan:
https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20TSAN/builds/10171

Original issue's description:
> [Turbofan] Clean up register allocator and verifier code.
>
> - Improves RegisterName display in register allocator.
> - Removes GetFixedRegisters method in preparation for having multiple fp
> reg types.
> - Clean up some verifier code.
>
> LOG=N
> BUG=v8:4124
>
> Committed: https://crrev.com/d99e1ab60557e86b62ccc6c91514e817c0df370d
> Cr-Commit-Position: refs/heads/master@{#37076}

TBR=mtrofin@chromium.org,bbudge@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:4124

Review-Url: https://codereview.chromium.org/2078243002
Cr-Commit-Position: refs/heads/master@{#37077}