add tests for fixed stage1 bugs

Closes ziglang#1957
Closes ziglang#1994
Closes ziglang#2140
Closes ziglang#2746
Closes ziglang#2802
Closes ziglang#2855
Closes ziglang#2895
Closes ziglang#2981
Closes ziglang#3054
Closes ziglang#3158
Closes ziglang#3234
Closes ziglang#3259
Closes ziglang#3371
Closes ziglang#3376
Closes ziglang#3387
Closes ziglang#3529
Closes ziglang#3653
Closes ziglang#3750
Closes ziglang#3778
Closes ziglang#3882
Closes ziglang#3915
Closes ziglang#3929
Closes ziglang#3961
Closes ziglang#3988
Closes ziglang#4123
Closes ziglang#7448

Author: Vexu

compile_errors/control_flow_uses_comptime_var_at_runtime.zig

@@ -6,10 +6,35 @@ export fn foo() void {
 }
 
 fn bar() void { }
+export fn baz() void {
+    comptime var idx: u32 = 0;
+    while (idx < 1) {
+        const not_null: ?u32 = 1;
+        _ = not_null orelse return;
+        idx += 1;
+    }
+}
+
+export fn qux() void {
+    comptime var i = 0;
+    while (i < 3) : (i += 1) {
+        const T = switch (i) {
+            0 => f32,
+            1 => i8,
+            2 => bool,
+            else => unreachable,
+        };
+        _ = T;
+    }
+}
 
 // error
 // backend=stage2
 // target=native
 //
 // :3:24: error: cannot store to comptime variable in non-inline loop
 // :3:5: note: non-inline loop here
+// :14:13: error: cannot store to comptime variable in non-inline loop
+// :11:5: note: non-inline loop here
+// :20:24: error: cannot store to comptime variable in non-inline loop
+// :20:5: note: non-inline loop here

compile_errors/reference_to_const_data.zig

@@ -18,6 +18,10 @@ export fn qux() void {
     var ptr = &S{.x=1,.y=2};
     ptr.x = 2;
 }
+export fn quux() void {
+    var x = &@returnAddress();
+    x.* = 6;
+}
 
 // error
 // backend=stage2
@@ -27,3 +31,4 @@ export fn qux() void {
 // :7:8: error: cannot assign to constant
 // :11:8: error: cannot assign to constant
 // :19:8: error: cannot assign to constant
+// :23:6: error: cannot assign to constant

compile_errors/return_incompatible_generic_struct.zig

@@ -0,0 +1,20 @@
+fn A(comptime T: type) type {
+    return struct { a: T };
+}
+fn B(comptime T: type) type {
+    return struct { b: T };
+}
+fn foo() A(u32) {
+    return B(u32){ .b = 1 };
+}
+export fn entry() void {
+    _ = foo();
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :8:18: error: expected type 'tmp.A(u32)', found 'tmp.B(u32)'
+// :5:12: note: struct declared here
+// :2:12: note: struct declared here