sema: remove special case check in is_non_err

test/cases/error_union_variant_is_runtime_known.zig

@@ -0,0 +1,22 @@
+// This tests that the variant of an error union is runtime-known when the value is runtime-known.
+// This might seem obvious but previously the compiler special-cased the situation where a const
+// was assigned a payload or error value, i.e. instead of another error union.
+
+export fn foo() void {
+    var runtime_payload: u8 = 0;
+    _ = &runtime_payload;
+    const eu: error{a}!u8 = runtime_payload;
+    if (eu) |_| {} else |_| @compileError("analyzed");
+}
+
+export fn bar() void {
+    var runtime_error: error{a} = error.a;
+    _ = &runtime_error;
+    const eu: error{a}!u8 = runtime_error;
+    if (eu) |_| @compileError("analyzed") else |_| {}
+}
+
+// error
+//
+// :9:29: error: analyzed
+// :16:17: error: analyzed