[clang] [llvm] [PowerPC] Add AMO load builtins for conditional increment/decrement (PR #169435)

[Lei Huang via cfe-commits]

https://github.com/lei137 approved this pull request.

LGTM.
Would be good if we can get a follow refactor for the sema error message handling.

https://github.com/llvm/llvm-project/pull/169435