The output was utterly confusing because block identities were printed
in different ways:

- "id:5" for a block with id 5
- "B5" for a block with rpo number 5
- also (!!!) "B5" for a block with id 5


With this CL, the last case above is eliminated such that there is no
ambiguity. I originally wanted to unify the prefix syntax as well (e.g.
"id:5" and "rpo:5"), but the prefixes are hard-coded in countless
places including CodeGenerator, Turbolizer, and Verifier. Many of these
are format strings that are painful to write more generically.

Change-Id: I0eb70731c7b1ef9a9999e0bcb58b673288932e93
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2940890
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75092}