See https://github.com/openjdk/valhalla/blob/b1d14c658511cced2a860d9e2741ae89827e25ba/src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp#L6040-L6073 for the shape of the stack there.

LR/FP#2 are not reliable:
- rfp can contain an oop, and the GC is only aware it is save in FP#1 and will only update this (after JDK-8367151)
- only LR#1 is patched when deoptimizing

so we should only use LR/FP#1. LR/FP#2 are only used as spacing. Rather than taking the risk of reading correct-looking values (but possibly not), in debug, we can save some clearly bad values that would never work by accident.