[flang] verification of lowering to FIR failed on an assigned GOTO using an allocatable label variable

Reproducer:
https://godbolt.org/z/vTvPPTqoa
```f90
allocatable :: l
assign 10 to l
goto l
10 continue
end
```

Output:
```console
error: loc("/app/example.f90":3:1): 'fir.select' op must be an integer
error: verification of lowering to FIR failed
Compiler returned: 1
```

The expected behavior is that the code either compiles successfully or produces an appropriate and informative diagnostic.