[WebAssembly] Fix a bug in MachineBasicBlock::findDebugLoc() call

[aheejin]

Summary

InsertPos is within the bacic block Header, so findDebugLoc() should be called on not MBB but Header instead.

[aheejin]

comment fix

[aheejin]

Delete findPrevDebugLoc() call and leave only Header bugfix

[yurydelendik]

This makes sense. Thank you for the patch.

lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp

while (InsertPos != Header->begin() &&
            WebAssembly::isChild(*std::prev(InsertPos), MFI) &&
            std::prev(InsertPos)->getOpcode() != WebAssembly::LOOP &&
            std::prev(InsertPos)->getOpcode() != WebAssembly::END_BLOCK &&
            std::prev(InsertPos)->getOpcode() != WebAssembly::END_LOOP)
       --InsertPos;
   }
 
   // Add the BLOCK.
-  MachineInstr *Begin = BuildMI(*Header, InsertPos, MBB.findDebugLoc(InsertPos),
-                                TII.get(WebAssembly::BLOCK))
-                            .addImm(int64_t(WebAssembly::ExprType::Void));
+  DebugLoc DL = InsertPos != Header->end()
+                    ? Header->findDebugLoc(InsertPos)
+                    : Header->findPrevDebugLoc(InsertPos);
+  MachineInstr *Begin =
+      BuildMI(*Header, InsertPos, DL, TII.get(WebAssembly::BLOCK))
 
   // Mark the end of the block.
   InsertPos = MBB.begin();
   while (InsertPos != MBB.end() &&
          InsertPos->getOpcode() == WebAssembly::END_LOOP &&
          LoopTops[&*InsertPos]->getParent()->getNumber() >= Header->getNumber())
     ++InsertPos;
   MachineInstr *End = BuildMI(MBB, InsertPos, MBB.findPrevDebugLoc(InsertPos),
static void PlaceLoopMarker(
   MachineBasicBlock *AfterLoop = &*Iter;
 
   // Mark the beginning of the loop (after the end of any existing loop that
   // ends here).
   auto InsertPos = MBB.begin();
   while (InsertPos != MBB.end() &&
          InsertPos->getOpcode() == WebAssembly::END_LOOP)
     ++InsertPos;
-  MachineInstr *Begin = BuildMI(MBB, InsertPos, MBB.findDebugLoc(InsertPos),
-                                TII.get(WebAssembly::LOOP))
+
+  DebugLoc DL = InsertPos != MBB.end() ? MBB.findDebugLoc(InsertPos)
+                                       : MBB.findPrevDebugLoc(InsertPos);
+  MachineInstr *Begin = BuildMI(MBB, InsertPos, DL, TII.get(WebAssembly::LOOP))
                             .addImm(int64_t(WebAssembly::ExprType::Void));
 
   // Mark the end of the loop (using arbitrary debug location that branched
   // to the loop end as its location).
   DebugLoc EndDL = (*AfterLoop->pred_rbegin())->findBranchDebugLoc();
   MachineInstr *End = BuildMI(*AfterLoop, AfterLoop->begin(), EndDL,
                               TII.get(WebAssembly::END_LOOP));
   LoopTops[End] = Begin;