Naga should permit override expressions as LocalVariable initializers

[jimblandy]

At present, Naga only allows constant expressions in LocalVariable::init, but there's no reason it couldn't permit override expressions as well.

jimblandy@7985ce1 seems to work fine.

[jimblandy]

Note that this is unrelated to WGSL compliance. The WGSL front end already accepts any sort of expression as an initializer for LocalVariables, but initialization by a runtime expression is handled via an explicit Store statement. The question here is merely which initializations we can render using LocalVariable::init, and which require Stores.

Using init produces slightly cleaner output.