Reapply "Fix error in unrecognized register name handling for...

Reapply "Fix error in unrecognized register name handling for "SBFram…e.register"" (#88468)" (#88535)

The only change is a fix for the "register" iterator test to not rely on
particular register names.

I mistook where the artificial "pc" register is generated. It isn't
added to the register list or the register sets (except on arm where
that's the name of the actual register), so I can't use it in this test.
I instead just assert that the "register" generator produces the same
list as flattening the register sets from "registers".

This reverts commit 9f149147.