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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> `PrivateLocalPrefix` was added in commit [4e27343](https://github.com/llvm/llvm-project/commit/4e27343eec284d9028617171a82a5816420fe4c7). @arsenm Is this still used?

The HSAIL backend isn't maintained anymore, so probably not 

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