[AArch64][LOH] Don't ignore regmasks in bundles by iterating over instrs.

The LOH pass iterates over instructions to build its custom register
state machine, but it uses the top-level bundle iterator.
This should be okay, because when the wrapper BUNDLE MI is built,
it aggregates the register defs/uses in its instructions into MOs.

However, that doesn't apply to regmasks, and accumulating regmasks
across multiple instructions would be messy business.
There are a couple AnalyzePhysRegInBundle (/Virt) helpers that
do look at regmasks, but those don't fit in very well here.

AArch64 has started to use a few bundle instructions, specifically
as glorified pseudos for variant call instructions, which have regmasks.
So the LOH pass ends up ignoring regmasks.

Concretely, this has been wrong for a while, but, on aarch64, the
most common bundle (rv_marker call) was always followed by the
attached call instruction, a plain BL with a regmask.  Which
was properly detected by the pass.

However, we recently started keeping the attached call in the bundle,
so the regmask is now ignored.  And the pass happily combines ADRPs, of
say, x8, across the bundle, resulting in corrupt pointers later.

Author: ahmedbougacha

llvm/lib/Target/AArch64/AArch64CollectLOH.cpp

@@ -559,7 +559,7 @@ bool AArch64CollectLOH::runOnMachineFunction(MachineFunction &MF) {
     // Walk the basic block backwards and update the per register state machine
     // in the process.
     for (const MachineInstr &MI :
-         instructionsWithoutDebug(MBB.rbegin(), MBB.rend())) {
+         instructionsWithoutDebug(MBB.instr_rbegin(), MBB.instr_rend())) {
       unsigned Opcode = MI.getOpcode();
       switch (Opcode) {
       case AArch64::ADDXri:

llvm/test/CodeGen/AArch64/loh.mir

@@ -190,4 +190,13 @@ body: |
   bb.14:
     liveins: $x10
     $x12 = ADDXri $x10, target-flags(aarch64-pageoff) @g0, 0
+
+  bb.15:
+    ; Should not produce a LOH when reg is clobbered by bundled regmask
+    ; CHECK-NOT: MCLOH_AdrpAdrp
+    $x8 = ADRP target-flags(aarch64-page) @g0
+    BUNDLE {
+      BL @extfunc, csr_aarch64_aapcs
+    }
+    $x8 = ADRP target-flags(aarch64-page) @g1
 ...