DSLX: bug - SIGSEGV in IR generation when token is returned in a tuple

Consider the following code:
```
pub proc foo {
init {()}
config() {()}

next(tok: token, state: ()) {
let tok = if true {
tok
} else {
tok
};
()
}
}
```

Trying to make IR out of it results in the following, which I presume is expected:
```
$ ir_converter_main ...

F0619 18:16:20.305308 1718188 ir_converter_main.cc:163] Check failed: ::absl::OkStatus() == (status) (OK vs. INVALID_ARGUMENT: Could not build IR: Expected sel.5 to not contain a token in its type; type is token
File: 0, Line: 20, Col: 18
Stack Trace:
0x5650217e5732: xls::BuilderBase::SetError()
0x5650217e5af7: xls::BuilderBase::CreateBValue()
0x5650217e8846: xls::BuilderBase::Select()
0x5650217e8978: xls::BuilderBase::Select()
```

---

Now change the code as follows:
```
pub proc foo {
init {()}
config() {()}

next(tok: token, state: ()) {
let (a, tok) = if true {
(false, tok)
} else {
(true, tok)
};
()
}
}
```

Trying to run IR converter results in a silent segmentation fault. To figure out what happens the user must enable VLOG, scroll up and take a note of that SetError:
```
I0619 18:20:12.926616 1718441 verifier.cc:2014] Verifying node: sel.9: (bits[1], token) = sel(literal.4, cases=[tuple.8, tuple.6], id=9, pos=[(0,37,23)])
I0619 18:20:12.926625 1718441 function_builder.cc:109] BuilderBase::SetError; msg: Expected sel.9 to not contain a token in its type; type is (bits[1], token); loc: [(0,37,23)]
I0619 18:20:12.932035 1718441 function_converter.cc:370] Define node 'if true { (false, tok) } else { (true, tok) }' (Conditional) to be (nil) @ xls/modules/dbe/encoder_lz4.x:38:24-42:10
I0619 18:20:12.932047 1718441 function_converter.cc:419] Setting node 'if true { (false, tok) } else { (true, tok) }' (0x5562c8ed89f0) to IR value (nil).
I0619 18:20:12.932051 1718441 function_converter.cc:408] Using node 'if true { (false, tok) } else { (true, tok) }' (0x5562c8ed89f0) as IR value (nil).
I0619 18:20:12.932054 1718441 function_converter.cc:344] Aliased node 'let (a, tok) = if true { (false, tok) } else { (true, tok) };' (Let) to be same as 'if true { (false, tok) } else { (true, tok) }' (Conditional): (nil)
I0619 18:20:12.932059 1718441 function_converter.cc:641] Walking level 1 index 0: `a`
[1] 1718441 segmentation fault (core dumped) bazel-out/k8-opt-exec-2B5CBBC6/bin/xls/dslx/ir_convert/ir_converter_main
```

While doing such things with `token` may be forbidden for now, XLS should report it more gracefully.