AstGen: avoid intermediate loads during field/array access

[Vexu]

There should probably be a check that the access chain ends in a symbol since other cases could still use the _val versions?

Closes#15685
Closes#15280
Closes#12963

[mlugg]

The AstGen change here causes "local variable is never mutated" errors to not be emitted when they should, because that error is based on when a local variable is never accessed with a ref result location. I'll let you decide how best to handle this - perhaps you could introduce a .pseudo_ref result location which acts like .ref but exists only as an optimization so doesn't mark a local variable as mutated.

[Rexicon226]

b97ae63 should be documented in langref?

[andrewrk]

This change needs to come with benchmarks, an analysis of the differences in ZIR, and an overview explanation.

[Vexu]

b97ae63 should be documented in langref?

It should but note that it is just implementing #4765 in a slightly different situation (&array_ptr.ptr).

[Vexu]

I can't think of a way to get this to work right now and the latent bugs its uncovering aren't making it any easier.