[AArch64ConditionalCompares] Ignore debug insts in findConvertibleCompare [7/10]

[vsk]

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.

[aemerson]

LGTM.

[vsk]

retitle, use prev_nodbg helper

llvm/lib/Target/AArch64/AArch64ConditionalCompares.cpp

if (!I->readsRegister(AArch64::NZCV)) {
     ++NumCmpTermRejs;
     LLVM_DEBUG(dbgs() << "Flags not used by terminator: " << *I);
     return nullptr;
   }
 
   // Now find the instruction controlling the terminator.
   for (MachineBasicBlock::iterator B = MBB->begin(); I != B;) {
     --I;
+    I = skipDebugInstructionsBackward(I, MBB->begin());
     assert(!I->isTerminator() && "Spurious terminator");
     switch (I->getOpcode()) {
     // cmp is an alias for subs with a dead destination register.
     case AArch64::SUBSWri:
     case AArch64::SUBSXri:
     // cmn is an alias for adds with a dead destination register.
     case AArch64::ADDSWri:
     case AArch64::ADDSXri:

llvm/test/CodeGen/AArch64/arm64-ccmp.ll

-; RUN: llc < %s -mcpu=cyclone -verify-machineinstrs -aarch64-enable-ccmp -aarch64-stress-ccmp | FileCheck %s
+; RUN: llc < %s -debugify-and-strip-all-safe -mcpu=cyclone -verify-machineinstrs -aarch64-enable-ccmp -aarch64-stress-ccmp | FileCheck %s
 target triple = "arm64-apple-ios"
 
 ; CHECK: single_same
 ; CHECK: cmp w0, #5
 ; CHECK-NEXT: ccmp w1, #17, #4, ne
 ; CHECK-NEXT: b.ne
 ; CHECK: %if.then
 ; CHECK: bl _foo