[AArch64ConditionalCompares] Ignore debug insts in findConvertibleCompare [8/14]

Summary:
Fix an issue where the presence of debug info could disable the ccmp
optimization due to findConvertibleCompare failing too early (the error
is "Can't create ccmp with multiple uses", where the "use" is a
DBG_VALUE inst).

Depends on D78151.

Reviewers: t.p.northover, paquette, aemerson

Subscribers: kristof.beyls, hiraditya, danielkiss, aprantl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78156