Segmentation fault compiling switch on `union(enum(comptime_int))` with multiple fields

[amp-59]

Zig Version

0.12.0-dev.983+78f2ae7f2

Steps to Reproduce and Observed Behavior

Compile example program with zig build-obj union_enum_comptime_int.zig
union_enum_comptime_int.zig:

const Tag = union(enum(comptime_int)) { a, b };

export fn entry(payload: Tag) void {
    switch (payload) {
        else => {},
    }
}

Actual result:

zig build-obj union_enum_comptime_int.zig
Segmentation fault

Expected Behavior

[Rexicon226]

Attempting to reproduce on 0.12.0-dev.1632+acf9de376 creates:

thread 286834 panic: index out of bounds: index 0, len 0
/home/dr/zig/src/codegen/llvm.zig:8709:34: 0x1860716 in airArg (zig)
        const arg_val = self.args[self.arg_index];
                                 ^
/home/dr/zig/src/codegen/llvm.zig:4956:51: 0x131dfe8 in genBody (zig)
                .arg            => try self.airArg(inst),
                                                  ^
/home/dr/zig/src/codegen/llvm.zig:1648:19: 0x1318638 in updateFunc (zig)
        fg.genBody(air.getMainBody()) catch |err| switch (err) {
                  ^
/home/dr/zig/src/link/Elf.zig:3139:70: 0x132c236 in updateFunc (zig)
    if (self.llvm_object) |llvm_object| return llvm_object.updateFunc(mod, func_index, air, liveness);
                                                                     ^
/home/dr/zig/src/link.zig:621:77: 0xfe5992 in updateFunc (zig)
            .elf   => return @fieldParentPtr(Elf,   "base", base).updateFunc(module, func_index, air, liveness),
                                                                            ^
/home/dr/zig/src/Module.zig:3377:37: 0xd5c28a in ensureFuncBodyAnalyzed (zig)
            comp.bin_file.updateFunc(mod, func_index, air, liveness) catch |err| switch (err) {