[WebAssembly] Fix PHI when relaying longjmps

[aheejin]

Summary

When doing Emscritpen EH, if SjLj is also enabled and used and if the
thrown exception has a possiblity being a longjmp instead of an
exception, we shouldn't swallow it; we should rethrow, or relay it. It
was done in D106525 and the code is here:
https://github.com/llvm/llvm-project/blob/8441a8eea8007b9eaaaabf76055949180a702d6d/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp#L858-L898

Here is the pseudocode of that part: (copied from comments)

if (%__THREW__.val == 0 || %__THREW__.val == 1)
  goto %tail
else
  goto %longjmp.rethrow

longjmp.rethrow: ;; This is longjmp. Rethrow it
  %__threwValue.val = __threwValue
  emscripten_longjmp(%__THREW__.val, %__threwValue.val);

tail: ;; Nothing happened or an exception is thrown
  ... Continue exception handling ...

If the current BB (where the invoke is created) has successors that
has the current BB as its PHI incoming node, now that has to change to
tail in the pseudocode, because tail is the latest BB that is
connected with the next BB, but this was missing.

[tlively]

"shoudln't" => "shouldn't" in the PR description.

llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp

if (NeedInvoke) {
         IRB.CreateCondBr(Or, Tail, RethrowBB);
         IRB.SetInsertPoint(RethrowBB);
         Value *ThrewValue = IRB.CreateLoad(IRB.getInt32Ty(), ThrewValueGV,
                                            ThrewValueGV->getName() + ".val");
         IRB.CreateCall(EmLongjmpF, {Threw, ThrewValue});
 
         IRB.CreateUnreachable();
         IRB.SetInsertPoint(Tail);
+        BB.replaceSuccessorsPhiUsesWith(&BB, Tail);
       }
 
       // Insert a branch based on __THREW__ variable
       Value *Cmp = IRB.CreateICmpEQ(Threw, getAddrSizeInt(&M, 1), "cmp");
       IRB.CreateCondBr(Cmp, II->getUnwindDest(), II->getNormalDest());
 
     } else {
       // This can't throw, and we don't need this invoke, just replace it with a

llvm/test/CodeGen/WebAssembly/lower-em-ehsjlj.ll

entry:
   invoke void @foo()
           to label %try.cont unwind label %lpad
 ; CHECK:    entry:
 ; CHECK:      %cmp.eq.one = icmp eq i32 %__THREW__.val, 1
 ; CHECK-NEXT: %cmp.eq.zero = icmp eq i32 %__THREW__.val, 0
 ; CHECK-NEXT: %or = or i1 %cmp.eq.zero, %cmp.eq.one
 ; CHECK-NEXT: br i1 %or, label %tail, label %longjmp.rethrow
 
+; CHECK: try.cont:
+; CHECK-NEXT:  %phi = phi i32 [ undef, %tail ], [ undef, %lpad ]
+; CHECK-NEXT:  ret void
+
 ; CHECK:    tail:
 ; CHECK-NEXT: %cmp = icmp eq i32 %__THREW__.val, 1
 ; CHECK-NEXT: br i1 %cmp, label %lpad, label %try.cont
 
 ; CHECK:    longjmp.rethrow:
 ; CHECK-NEXT: %__threwValue.val = load i32, i32* @__threwValue, align 4
 ; CHECK-NEXT: call void @emscripten_longjmp(i32 %__THREW__.val, i32 %__threwValue.val)
 ; CHECK-NEXT: unreachable
 
 lpad:                                             ; preds = %entry
   %0 = landingpad { i8*, i32 }
           catch i8* null
   %1 = extractvalue { i8*, i32 } %0, 0
   %2 = extractvalue { i8*, i32 } %0, 1
   %3 = call i8* @__cxa_begin_catch(i8* %1) #5
   call void @__cxa_end_catch()
   br label %try.cont
 
 try.cont:                                         ; preds = %entry, %lpad
+ %phi = phi i32 [ undef, %entry ], [ undef, %lpad ]
   ret void
 }
 
 ; This function contains a setjmp call and no invoke, so we only handle longjmp
 ; here. But @foo can also throw an exception, so we check if an exception is
 ; thrown and if so rethrow it by calling @__resumeException. Also we have to
 ; free the setjmpTable buffer before calling @__resumeException.
 define void @rethrow_exception() {