This prints nicer error messages for checks like
"DCHECK_EQ(reg1, reg2)", and also splits cache state tracing into
one method for printing the overall state, one for printing each slot,
and one for printing the register.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I36e83ba2542986dd8ad17dbfe7cbb8df54a56755
Reviewed-on: https://chromium-review.googlesource.com/853495
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50407}