[llvm] [CodeGen] Use temp symbol for MBBs (PR #95031)

[via llvm-commits]

aengelke wrote:

> Shall we consider this blocked by the resolution to BPF https://github.com/llvm/llvm-project/pull/95103 ?

I don't think so. This maintains the existing functionality of separate label/global prefixes. Removing this distinction and cleaning up the code can be done later once all BPF also uses the same local/global prefix.

The only BPF-related change is that basic block labels are no longer in the symbol table if they are not referenced by inline assembly. That should be fine, because @4ast said in #95103:

> >  basic block labels would no longer be added to the symbol table
>
> that's a regression. they didn't meant to be in symbol table.

So this PR reduces this regression (although it does not fix it entirely, that's what #95103 originally was).

https://github.com/llvm/llvm-project/pull/95031