Tcheck: treat trivially false range checks as errors

[alastairreid]

Tcheck: treat trivially false range checks as errors

This makes expressions like '0b1[3:0]' or '3 / 0' into typechecking errors instead of just runtime errors.

Note that this only applies when

1. Expressions only involve literal constants.

   e.g. 'x > x+1' or '0 <= x && x < 0' will be checked at runtime.

2. Assertions that are automatically inserted by the compiler
   to check for errors like

   • x / 0
   • x / y when x or y is negative
   • my_array[i] when i is outside the bounds of the array
   • b[i], b[i +: 8], b[i -: 8], b[i *: 8] when those accesses
     go outside the bounds of the bitvector
   • let x : {0..7} := 15; // i.e., assigning an out of bounds value

3. Runtime error checks are enabled (using --runtime-errors)

So this does not guarantee to detect all errors.
In fact, it will miss quite a lot of possible errors.

But it will catch completely silly errors and, in particular, it will catch errors resulting from the forthcoming change to treat hex numbers as bitvectors that has resulted in a lot of expressions of the form "0x1[31:0]" creating latent runtime errors.