[enhancement][dslx] For a width slice with a const immediate start position >= size, give a warning

### What's hard to do? (limit 100 words)

Right now when imm is a u32 you can do imm[32 +: u32] and it will not flag that this is likely an error. This seems like a good warning to have in the DSLX warning set.

### Current best alternative workaround (limit 100 words)

Being cautious about this error pattern.

### Your view of the "best case XLS enhancement" (limit 100 words)

We give a warning for this case.