[WebAssembly] Fix PHI when relaying longjmps

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.

Reviewed By: tlively

Differential Revision: https://reviews.llvm.org/D108785

llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp

@@ -891,6 +891,7 @@ bool WebAssemblyLowerEmscriptenEHSjLj::runEHOnFunction(Function &F) {
 
         IRB.CreateUnreachable();
         IRB.SetInsertPoint(Tail);
+        BB.replaceSuccessorsPhiUsesWith(&BB, Tail);
       }
 
       // Insert a branch based on __THREW__ variable

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

@@ -65,6 +65,10 @@ entry:
 ; 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
@@ -84,6 +88,7 @@ lpad:                                             ; preds = %entry
   br label %try.cont
 
 try.cont:                                         ; preds = %entry, %lpad
+ %phi = phi i32 [ undef, %entry ], [ undef, %lpad ]
   ret void
 }