[clang] [llvm] [clang] Add more test coverage for "zero-call-used-regs" (PR #201834)

[Nick Desaulniers via cfe-commits]

https://github.com/nickdesaulniers commented:

LGTM; small cleanup possible in one of the unit tests. Thanks for the clear explanation for the order of events, and sorry I missed that in code review the first time around.

https://github.com/llvm/llvm-project/pull/201834