stage2: fix hashing and comparison design flaw with Value

 * `Value.toType` accepts a buffer parameter instead of an allocator
   parameter and can no longer fail.
 * Module: remove the unused `mod: *Module` parameter from various
   functions.
 * `Value.compare` now accepts a `Type` parameter which indicates the
   type of both operands. There is also a `Value.compareHetero` which
   accepts only Value parameters and supports comparing mixed types.
   Likewise, `Value.eql` requires a `Type` parameter.
 * `Value.hash` is removed; instead the hash map context structs now
   have a `ty: Type` field, and the hash function lives there, where it
   has access to a Value's Type when it computes a hash.
   - This allowed the hash function to be greatly simplified and sound
     in the sense that the same Values, even with different
     representations, always hash to the same thing.
 * Sema: Fix source location of zirCmp when an operand is runtime known
   but needs to be comptime known.
 * Remove unused target parameter from `Value.floatCast`.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/Air.zig b/src/Air.zig
index fb95d60d00..d202c079bc 100644
--- a/src/Air.zig
+++ b/src/Air.zig
@@ -503,7 +503,8 @@ pub fn typeOfIndex(air: Air, inst: Air.Inst.Index) Type {
 pub fn getRefType(air: Air, ref: Air.Inst.Ref) Type {
     const ref_int = @enumToInt(ref);
     if (ref_int < Air.Inst.Ref.typed_value_map.len) {
-        return Air.Inst.Ref.typed_value_map[ref_int].val.toType(undefined) catch unreachable;
+        var buffer: Value.ToTypeBuffer = undefined;
+        return Air.Inst.Ref.typed_value_map[ref_int].val.toType(&buffer);
     }
     const inst_index = ref_int - Air.Inst.Ref.typed_value_map.len;
     const air_tags = air.instructions.items(.tag);