[llvm] [ARM] Fix llvm.returnaddress for Thumb1 with R11 frame-pointer (PR #117735)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic approved this pull request.

For the pseudo, I was thinking it would be sort of opportunistic: if the return address happens to be available in LR, then mov from LR, else load it from the stack.  (If it's not in LR, it's almost always going to be somewhere on the stack.)  I guess that's potentially less efficient in some cases, but probably fine for Thumb1.

In any case, LGTM.

https://github.com/llvm/llvm-project/pull/117735