[llvm] [DWARF] Emit a worst-case prologue_end flag for pathological inputs (PR #107849)

[Jeremy Morse via llvm-commits]

jmorse wrote:

@labath much appreciated, I've got a rebased reapplication in #116084 where I'll use the CI to check this passes,

@hokein would there be any further context than that -- presumably UBSan can produce a stack trace? There are a lot of references to `MachineInstr` here, and not a lot to go on.

https://github.com/llvm/llvm-project/pull/107849