[llvm] [ARM] Avoid reference into modified vector (PR #93965)

[Rodolfo Wottrich via llvm-commits]

rgwott wrote:

Hi. This PR broke some tests downstream for us, using the ARM backend.
>From my debugging, it seems that the new &FirstCand in ARMBaseInstrInfo.cpp:6072 may refer to an empty container, thus breaking the subsequent access in line 6073. In the broken test case, this happens because CandidatesWithoutStackFixups in line 6061 can be empty.
I do not have a reproducer right now but will try to get one.
We did not observe problems with AArch64 yet, but we also did not investigate.
Thanks in advance for looking into this.

https://github.com/llvm/llvm-project/pull/93965