[naga] Validation relies on ensure_blocks_return pass to catch non-void functions which don't return a value

[jamienicol]

Description
Noticed this while working on #4395 / #7013. The problem was a non-void function which doesn't have a return statement. This typically gets caught by the validator here as a InvalidReturnType error. Note, however, that this just says "for each return statement, ensure that the type being returned matches the function's return type". It does not ensure that every control flow branch of a function eventually leads to a return statement.

This is fine as long as the front-end calls the ensure_block_returns() pass (modulo any bugs in that pass such as #4395), as we will then always have a return statement. But it feels very fragile for the validator to rely on a pass that a frontend may forget to call.

If we add a pass to the validator to check this properly, perhaps we can remove the ensure_block_returns pass altogether. This looks like it was added because SPIRV requires functions to end in a return statement (see gfx-rs/naga#301) but we handle that in the spv backend here. Though the other arguments given in that issue may still apply

[jamienicol]

I initially wrote in the description that the HLSL frontend does not call ensure_block_returns, but there is no HLSL frontend 😆 . I've edited the description to keep it accurate