This is inspired by the original variant of D109749 by Graham Hunter,
but is a more general version.
Roughly, instead of promoting the alloca, we call it a shadow/backing alloca,
go through all it's slices, clone(!) instructions that operated on it,
but make them operate on the cloned alloca, and promote cloned alloca instead.
This keeps the shadow/backing alloca, and all the original instructions around,
which results in said shadow/backing alloca being a perfect mirror/representation
of the promoted alloca's content, so calls that take the alloca
as arguments (non-capturingly!) can be supported.
For now, we require that the calls also don't modify the alloca's content,
but that is only to simplify the initial implementation,
and that will be supported in a follow-up.
Overall, this leads to *smaller* codesize:
https://llvm-compile-time-tracker.com/compare.php?from=a8b4f5bbab62091835205f3d648902432a4a5b58&to=aeae054055b125b011c1122f82c86457e159436f&stat=size-total
and is roughly neutral compile-time wise:
https://llvm-compile-time-tracker.com/compare.php?from=a8b4f5bbab62091835205f3d648902432a4a5b58&to=aeae054055b125b011c1122f82c86457e159436f&stat=instructions
I guess ; is unintentional here.