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.

1 files changed, 0 insertions, 2 deletions

diff --git a/test/behavior/eval.zig b/test/behavior/eval.zig
index f14a25451d..220768c820 100644
--- a/test/behavior/eval.zig
+++ b/test/behavior/eval.zig
@@ -1191,8 +1191,6 @@ test "no dependency loop for alignment of self tagged union" {
 }
 
 test "equality of pointers to comptime const" {
-    if (builtin.zig_backend != .stage1) return error.SkipZigTest; // TODO
-
     const a: i32 = undefined;
     comptime assert(&a == &a);
 }