AstGen: make sure for range start and end are usizes

Fixes #16311

The actual cause of #16311 is the `start_is_zero` special case:
```zig
                const range_len = if (end_val == .none or start_is_zero)
                    end_val
                else
                    try parent_gz.addPlNode(.sub, input, Zir.Inst.Bin{
                        .lhs = end_val,
                        .rhs = start_val,
                    });
```
It only happens if the range start is 0. In that case we would not perform any type checking.
Only in the other cases coincidentally `.sub` performs type checking in Sema, but the errors are still rather poor:
```
$ zig test x.zig
x.zig:9:15: error: invalid operands to binary expression: 'Pointer' and 'Pointer'
    for ("abc".."def") |val| {
         ~~~~~^~~~~~~
```
Note how it's the same as if I use `-`:
```
x.zig:9:11: error: invalid operands to binary expression: 'Pointer' and 'Pointer'
    "abc" - "def";
    ~~~~~~^~~~~~~
```
Now after this PR, the errors are much clearer for both range start and end:
```
x.zig:9:10: error: expected type 'usize', found '*const [3:0]u8'
    for ("abc".."def") |val| {
         ^~~~~
```
This is why I decided to use `.ty` instead of `.coerced_ty` for both range start and end rather than
just perform type checking in that `end_val == .none or start_is_zero` case.

0 files changed, 0 insertions, 0 deletions