[AArch64][Falkor] Fix bug in falkor prefetcher fix pass.

[gberry]

Summary

In rare cases, loads that don't get prefetched that were marked as
strided loads could cause a crash if they occurred in a loop with other
colliding loads.

[mcrosier]

LGTM.

lib/Target/AArch64/AArch64FalkorHWPFFix.cpp

void FalkorHWPFFix::runOnLoop(MachineLoop &L, MachineFunction &Fn) {
   for (MachineBasicBlock *MBB : L.getBlocks()) {
     LR.clear();
     LR.addLiveOuts(*MBB);
     for (auto I = MBB->rbegin(); I != MBB->rend(); LR.stepBackward(*I), ++I) {
       MachineInstr &MI = *I;
       if (!TII->isStridedAccess(MI))
         continue;
 
-      LoadInfo LdI = *getLoadInfo(MI);
-      unsigned OldTag = *getTag(TRI, MI, LdI);
-      auto &OldCollisions = TagMap[OldTag];
+      Optional<LoadInfo> OptLdI = getLoadInfo(MI);
+      if (!OptLdI)
+        continue;
+      LoadInfo LdI = *OptLdI;
+      Optional<unsigned> OptOldTag = getTag(TRI, MI, LdI);
+      if (!OptOldTag)
+        continue;
+      auto &OldCollisions = TagMap[*OptOldTag];
       if (OldCollisions.size() <= 1)
         continue;
 
       bool Fixed = false;
       DEBUG(dbgs() << "Attempting to fix tag collision: " << MI);
 
       for (unsigned ScratchReg : AArch64::GPR64RegClass) {
         if (!LR.available(ScratchReg) || MRI.isReserved(ScratchReg))

test/CodeGen/AArch64/falkor-hwpf-fix.mir

bb.0:
 
     %w0 = SUBWri %w0, 1, 0
     %wzr = SUBSWri %w0, 0, 0, implicit-def %nzcv
     Bcc 9, %bb.0, implicit %nzcv
 
   bb.1:
     RET_ReallyLR
 ...
+---
+# Check that we handle case of strided load with no HW prefetcher tag correctly.
+
+# CHECK-LABEL: name: hwpf_notagbug
+# CHECK-NOT: ORRXrs %xzr
+# CHECK: LDARW %x1
+# CHECK-NOT: ORRXrs %xzr
+# CHECK: LDRWui %x1
+name:            hwpf_notagbug
+tracksRegLiveness: true
+body: |
+  bb.0:
+    liveins: %w0, %x1, %x17
+
+    %w1 = LDARW %x1 :: ("aarch64-strided-access" load 4)
+    %w1 = LDRWui %x1, 0 :: ("aarch64-strided-access" load 4)
+    %w17 = LDRWui %x17, 0 :: ("aarch64-strided-access" load 4)
+
+    %w0 = SUBWri %w0, 1, 0
+    %wzr = SUBSWri %w0, 0, 0, implicit-def %nzcv
+    Bcc 9, %bb.0, implicit %nzcv
+
+  bb.1:
+    RET_ReallyLR
+...