Sema: clean up peer resolution of errors

 * Fix compile error for `zirErrorUnionType`.
 * Convert zirMergeErrorSets logic to call `Type.errorSetMerge`.
   It does not need to create a Decl as the TODO comment hinted.
 * Extract out a function called `resolveInferredErrorSetTy`.
 * Rework `resolvePeerTypes` with respect to error unions and
   error sets. This is a less complex implementation that passes all the
   same tests and uses many fewer lines of code by taking advantage of
   the function `coerceInMemoryAllowedErrorSets`.
   - Always merge error sets in the order that makes sense, even when
     that means `@typeInfo` incompatibility with stage1.
 * `Type.errorSetMerge` no longer overallocates.
 * Don't skip passing tests.

1 files changed, 1 insertions, 4 deletions

diff --git a/test/behavior/error.zig b/test/behavior/error.zig
index 582a11ebc4..12cc3983a9 100644
--- a/test/behavior/error.zig
+++ b/test/behavior/error.zig
@@ -264,11 +264,8 @@ fn testErrToIntWithOnePossibleValue(
 }
 
 test "error union peer type resolution" {
-    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
-    if (builtin.zig_backend == .stage2_c) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
 
     try testErrorUnionPeerTypeResolution(1);
 }