Co-authored-by: Jim Blandy <jimb@red-bean.com>

… declarations

There's no need to build a fresh `ConstantEvaluator` for every
expression; just build it once and reuse it.

This removes some clones and collects, simplifies call sites, and
isn't any more complicated to implement.

I found I needed a little bit more detail here.

Properly adjust `AtomicFunction::Exchange::compare` after pipeline
constant evaluation.

… API

Allow `LocalVariable::init` to be an override expression.

Note that this is unrelated to WGSL compliance. The WGSL front end
already accepts any sort of expression as an initializer for
`LocalVariable`s, but initialization by an override expression was
handled in the same way as initialization by a runtime expression, via
an explicit `Store` statement.

This commit merely lets us skip the `Store` when the initializer is an
override expression, producing slightly cleaner output in some cases.