`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.
3c8f3b91d8