Naga SPIR-V backend assumes that let-bound indexes are bounds checked in validation

[jimblandy]

As shown in the example in #4390, WGSL says that let-bound identifiers are not constants, and thus bounds checks cannot cause validation errors, but must be handled at runtime. But the SPIR-V backend includes code that assumes that any index known at compile time must have been checked already, and can be assumed to be in bounds.

When #4390 is fixed, the SPIR-V backend will need to be updated.

It might be good to search for all uses of these functions:

• GuardedIndex::from_expression
• GuardedIndex::try_resolve_to_constant