[AArch64ConditionOptimizer] Fix missed optimization due to debug insts [11/14]

Summary:
The findSuitableCompare method can fail if debug instructions are
present in the MBB -- fix this by using helpers to skip over debug
insts.

Reviewers: aemerson, paquette

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78265