[WebAssembly] Treat 'rethrow' as terminator in custom isel (llvm#95967)

`rethrow` instruction is a terminator, but when when its DAG is built in
`SelectionDAGBuilder` in a custom routine, it was NOT treated as such.

```ll
rethrow:                                          ; preds = %catch.start
  invoke void @llvm.wasm.rethrow() #1 [ "funclet"(token %1) ]
          to label %unreachable unwind label %ehcleanup

ehcleanup:                                        ; preds = %rethrow, %catch.dispatch
  %tmp = phi i32 [ 10, %catch.dispatch ], [ 20, %rethrow ]
  ...
```

In this bitcode, because of the `phi`, a `CONST_I32` will be created in
the `rethrow` BB. Without this patch, the DAG for the `rethrow` BB looks
like this:
```
  t0: ch,glue = EntryToken
      t3: ch = CopyToReg t0, Register:i32 %9, Constant:i32<20>
      t5: ch = llvm.wasm.rethrow t0, TargetConstant:i32<12161>
    t6: ch = TokenFactor t3, t5
  t8: ch = br t6, BasicBlock:ch<unreachable 0x562532e43c50>
```
Note that `CopyToReg` and `llvm.wasm.rethrow` don't have dependence so
either can come first in the selected code, which can result in the code
like
```mir
bb.3.rethrow:
  RETHROW 0, implicit-def dead $arguments
  %9:i32 = CONST_I32 20, implicit-def dead $arguments
  BR %bb.6, implicit-def dead $arguments
```

After this patch, `llvm.wasm.rethrow` is treated as a terminator, and
the DAG will look like
```
        t0: ch,glue = EntryToken
      t3: ch = CopyToReg t0, Register:i32 %9, Constant:i32<20>
    t5: ch = llvm.wasm.rethrow t3, TargetConstant:i32<12161>
  t7: ch = br t5, BasicBlock:ch<unreachable 0x5555e3d32c70>
```
Note that now `rethrow` takes a token from `CopyToReg`, so `rethrow` has
to come after `CopyToReg`. And the resulting code will be
```mir
bb.3.rethrow:
  %9:i32 = CONST_I32 20, implicit-def dead $arguments
  RETHROW 0, implicit-def dead $arguments
  BR %bb.6, implicit-def dead $arguments
```

I'm not very familiar with the internals of `getRoot` vs.
`getControlRoot`, but other terminator instructions seem to use the
latter, and using it for `rethrow` too worked.

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -3355,7 +3355,7 @@ void SelectionDAGBuilder::visitInvoke(const InvokeInst &I) {
       // special because it can be invoked, so we manually lower it to a DAG
       // node here.
       SmallVector<SDValue, 8> Ops;
-      Ops.push_back(getRoot()); // inchain
+      Ops.push_back(getControlRoot()); // inchain for the terminator node
       const TargetLowering &TLI = DAG.getTargetLoweringInfo();
       Ops.push_back(
           DAG.getTargetConstant(Intrinsic::wasm_rethrow, getCurSDLoc(),

llvm/test/CodeGen/WebAssembly/exception-legacy.ll

@@ -350,8 +350,60 @@ ehcleanupret:                                     ; preds = %catch.start, %ehcle
   cleanupret from %0 unwind to caller
 }
 
+; Regression test for the bug that 'rethrow' was not treated correctly as a
+; terminator in isel.
+define void @test_rethrow_terminator() personality ptr @__gxx_wasm_personality_v0 {
+entry:
+  invoke void @foo()
+          to label %try.cont unwind label %catch.dispatch
+
+catch.dispatch:                                   ; preds = %entry
+  %0 = catchswitch within none [label %catch.start] unwind label %ehcleanup
+
+catch.start:                                      ; preds = %catch.dispatch
+  %1 = catchpad within %0 [ptr @_ZTIi]
+  %2 = call ptr @llvm.wasm.get.exception(token %1)
+  %3 = call i32 @llvm.wasm.get.ehselector(token %1)
+  %4 = call i32 @llvm.eh.typeid.for.p0(ptr @_ZTIi)
+  %matches = icmp eq i32 %3, %4
+  br i1 %matches, label %catch, label %rethrow
+
+catch:                                            ; preds = %catch.start
+  %5 = call ptr @__cxa_begin_catch(ptr %2) [ "funclet"(token %1) ]
+  %6 = load i32, ptr %5, align 4
+  call void @__cxa_end_catch() [ "funclet"(token %1) ]
+  catchret from %1 to label %try.cont
+
+rethrow:                                          ; preds = %catch.start
+  invoke void @llvm.wasm.rethrow() #1 [ "funclet"(token %1) ]
+          to label %unreachable unwind label %ehcleanup
+
+try.cont:                                         ; preds = %entry, %catch
+  ret void
+
+ehcleanup:                                        ; preds = %rethrow, %catch.dispatch
+  ; 'rethrow' BB is this BB's predecessor, and its
+  ; 'invoke void @llvm.wasm.rethrow()' is lowered down to a 'RETHROW' in Wasm
+  ; MIR. And this 'phi' creates 'CONST_I32' instruction in the predecessor
+  ; 'rethrow' BB. If 'RETHROW' is not treated correctly as a terminator, it can
+  ; create a BB like
+  ; bb.3.rethrow:
+  ;   RETHROW 0
+  ;   %0 = CONST_I32 20
+  ;   BR ...
+  %tmp = phi i32 [ 10, %catch.dispatch ], [ 20, %rethrow ]
+  %7 = cleanuppad within none []
+  call void @take_i32(i32 %tmp) [ "funclet"(token %7) ]
+  cleanupret from %7 unwind to caller
+
+unreachable:                                      ; preds = %rethrow
+  unreachable
+}
+
+
 declare void @foo()
 declare void @bar(ptr)
+declare void @take_i32(i32)
 declare i32 @__gxx_wasm_personality_v0(...)
 ; Function Attrs: noreturn
 declare void @llvm.wasm.throw(i32, ptr) #1