[WebAssembly] Handle END_LOOP in unreachable BB in CFGStackify

[aheejin]

Summary

This fixes crashes when a BB in which an END_LOOP is to be placed is
unreachable and does not have any predecessors. Fixes PR41307.

[aheejin]

Fix test name

[aheejin]

• Test comment

lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp

for (const auto &MI : MBB)
     // Existing END_LOOP markers belong to parent loops of this loop
     if (MI.getOpcode() == WebAssembly::END_LOOP)
       AfterSet.insert(&MI);
 #endif
 
   // Mark the end of the loop (using arbitrary debug location that branched to
   // the loop end as its location).
   InsertPos = getEarliestInsertPos(AfterLoop, BeforeSet, AfterSet);
-  DebugLoc EndDL = (*AfterLoop->pred_rbegin())->findBranchDebugLoc();
+  DebugLoc EndDL = AfterLoop->pred_empty()
+                       ? DebugLoc()
+                       : (*AfterLoop->pred_rbegin())->findBranchDebugLoc();
   MachineInstr *End =
       BuildMI(*AfterLoop, InsertPos, EndDL, TII.get(WebAssembly::END_LOOP));
   registerScope(Begin, End);
 
   assert((!ScopeTops[AfterLoop->getNumber()] ||
           ScopeTops[AfterLoop->getNumber()]->getNumber() < MBB.getNumber()) &&
          "With block sorting the outermost loop for a block should be first.");
   if (!ScopeTops[AfterLoop->getNumber()])

test/CodeGen/WebAssembly/cfg-stackify-dbg.mir

+# RUN: llc -mtriple=wasm32-unknown-unknown -run-pass wasm-cfg-stackify %s -o - | FileCheck %s
+
+# This tests if END_LOOP marker is placed correctly when the BB it is to be
+# placed is unreachable.
+
+# CHECK-LABEL: unreachable_test
+name: unreachable_test
+liveins:
+  - { reg: '$arguments' }
+body: |
+  bb.0:
+    successors: %bb.1
+
+  bb.1:
+  ; predecessors: %bb.0, %bb.2
+    successors: %bb.2
+
+  bb.2:
+  ; predecessors: %bb.1
+    successors: %bb.1
+    BR %bb.1, implicit-def dead $arguments
+
+  ; CHECK: bb.3
+  ; CHECK: END_LOOP
+  bb.3:
+    UNREACHABLE implicit-def $arguments
+...