AstGen: avoid redundant "ref" instructions

Whenever a `ref` instruction is needed, it is created and saved in
`AstGen.ref_table` instead of being immediately appended to the current
block body. Then, when the referenced instruction is being added to the
parent block (e.g. from setBlockBody), if it has a ref_table entry, then
the ref instruction is added directly after the instruction being referenced.
This makes sure two properties are upheld:
1. All pointers to the same locals return the same address. This is required
   to be compliant with the language specification.
2. `ref` instructions will dominate their uses. This is a required property
   of ZIR.

A complication arises when a ref instruction refs another ref
instruction. The logic in appendBodyWithFixups must take this into
account, recursively handling ref refs.

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