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

prologue_end usually indicates where the end of the function-initialization
lies, and is where debuggers usually choose to put the initial breakpoint
for a function. Our current algorithm piggy-backs it on the first available
source-location: which doesn't necessarily have anything to do with the
start of the function.

To avoid this in heavily-optimised code that lacks many useful source
locations, pick a worst-case "if all else fails" prologue_end location, of
the first instruction that appears to do meaningful computation. It'll be
given the function-scope line number, which should run-on from the start of
the function anyway. This means if your code is completely inverted by the
optimiser, you can at least put a breakpoint at the _start_ like you
expect, even if it's difficult to then step through.

This patch also attempts to preserve some good behaviour we have without
optimisations -- at O0, if the prologue immediately falls into a loop body
without any computation happening, then prologue_end lands at the start of
that loop. This is desirable; but does mean we need to do more work to
detect and support those situations.

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