stage2: fix global variables with inferred type

Also, when a global variable does have a type, perform coercion on it.

Author: andrewrk

src/Sema.zig

@@ -7920,7 +7920,6 @@ fn zirVarExtended(
     const mut_src: LazySrcLoc = src; // TODO add a LazySrcLoc that points at mut token
     const init_src: LazySrcLoc = src; // TODO add a LazySrcLoc that points at init expr
     const small = @bitCast(Zir.Inst.ExtendedVar.Small, extended.small);
-    const var_ty = try sema.resolveType(block, ty_src, extra.data.var_type);
 
     var extra_index: usize = extra.end;
 
@@ -7940,11 +7939,25 @@ fn zirVarExtended(
     //    break :blk align_tv.val;
     //} else Value.initTag(.null_value);
 
-    const init_val: Value = if (small.has_init) blk: {
+    const uncasted_init: Air.Inst.Ref = if (small.has_init) blk: {
         const init_ref = @intToEnum(Zir.Inst.Ref, sema.code.extra[extra_index]);
         extra_index += 1;
-        const init_air_inst = sema.resolveInst(init_ref);
-        break :blk (try sema.resolveMaybeUndefVal(block, init_src, init_air_inst)) orelse
+        break :blk sema.resolveInst(init_ref);
+    } else .none;
+
+    const have_ty = extra.data.var_type != .none;
+    const var_ty = if (have_ty)
+        try sema.resolveType(block, ty_src, extra.data.var_type)
+    else
+        sema.typeOf(uncasted_init);
+
+    const init_val = if (uncasted_init != .none) blk: {
+        const init = if (have_ty)
+            try sema.coerce(block, var_ty, uncasted_init, init_src)
+        else
+            uncasted_init;
+
+        break :blk (try sema.resolveMaybeUndefVal(block, init_src, init)) orelse
             return sema.failWithNeededComptime(block, init_src);
     } else Value.initTag(.unreachable_value);
 

test/behavior/atomics.zig

@@ -111,3 +111,10 @@ fn test_u128_cmpxchg() !void {
     try expect(@cmpxchgStrong(u128, &x, 5678, 42, .SeqCst, .SeqCst) == null);
     try expect(x == 42);
 }
+
+var a_global_variable = @as(u32, 1234);
+
+test "cmpxchg on a global variable" {
+    _ = @cmpxchgWeak(u32, &a_global_variable, 1234, 42, .Acquire, .Monotonic);
+    try expect(a_global_variable == 42);
+}

test/behavior/atomics_stage1.zig

@@ -3,13 +3,6 @@ const expect = std.testing.expect;
 const expectEqual = std.testing.expectEqual;
 const builtin = @import("builtin");
 
-var a_global_variable = @as(u32, 1234);
-
-test "cmpxchg on a global variable" {
-    _ = @cmpxchgWeak(u32, &a_global_variable, 1234, 42, .Acquire, .Monotonic);
-    try expectEqual(@as(u32, 42), a_global_variable);
-}
-
 test "atomic load and rmw with enum" {
     const Value = enum(u8) {
         a,