BUG: Unexpected comparison X.B; with 1

[vlstill]

Seems like we crash when switch case contains a serializable enum value.

Related to p4lang/p4-spec#1369, #5246.

<stdin>(64): [--Wwarn=mismatch] warning: 32w1: constant expression in switch
        switch (32w1) {
                ^^^^
<stdin>(17): [--Wwarn=parser-transition] warning: SelectCase: unreachable case
            2: two;
            ^^^^^^
<stdin>(37): [--Wwarn=parser-transition] warning: accept state in parser p1 is unreachable
parser p1(out bit<32> v) {
       ^^
Compiler bug triggered, please report this error:
exception type: Util::CompilerBug
error: In file: ../../../p4c/frontends/p4/simplifySwitch.cpp:27
Compiler Bug: <stdin>(66): Unexpected comparison X.B; with 1
            X.B: { v = v + 20; }
            ^^^
<stdin>(64)
        switch (32w1) {
                ^^^^

Modified p4c/testdata/p4_16_samples/issue2617.p4

#include <core.p4>
enum bit<32> X {
    A = 0,
    B = 1
}

enum E {
    A, B, C
}

parser p(out bit<32> v) {
    state start {
        transition select (32w1) {
            0: zero;
            1: one;
            2: two;
        }
    }

    state zero {
        v = 0;
        transition accept;
    }

    state one {
        v = 1;
        transition accept;
    }

    state two {
        v = 2;
        transition reject;
    }
}

parser p1(out bit<32> v) {
    state start {
        transition select (X.A, E.A, 32w0) {
            (X.A, E.B, _): zero;
            (X.A, E.A, 1): one;
            (X.A, _, _): two;
        }
    }

    state zero {
        v = 0;
        transition accept;
    }

    state one {
        v = 1;
        transition accept;
    }

    state two {
        v = 2;
        transition reject;
    }
}

control c(out bit<32> v) {
    apply {
        switch (32w1) {
            0: { v = 0; }
            X.B: { v = v + 20; }
        }
    }
}

parser _p(out bit<32> v);
control _c(out bit<32> v);
package top(_p _p0, _p p1, _c _c0);

top(p(), p1(), c()) main;

[ChrisDodd]

Only when the switch expression is a constant. There's also an issue when the switch expression is a directionless arg.