fix comptime @ptrcasting from a larger type to a smaller one (#31774)

closes #30180

Note from mlugg: this fix is very much a hack, but it definitely won't break anything and it demonstrably fixes one case, so I'm merging it for now with the expectation that I'll be replacing the broken code soon.

Reviewed-on: https://codeberg.org/ziglang/zig/pulls/31774
Reviewed-by: mlugg <mlugg@noreply.codeberg.org>
Co-authored-by: glowsquid <sachabarsayuracko@gmail.com>
Co-committed-by: glowsquid <sachabarsayuracko@gmail.com>

test/behavior/ptrcast.zig

@@ -562,3 +562,11 @@ test "@ptrCast array pointer removing sentinel" {
     comptime assert(out[2] == 3);
     comptime assert(out[3] == 4);
 }
+
+test "@ptrcast larger type to smaller one" {
+    const T = packed struct { x: u17 };
+    const a: u32 = 0;
+    const b: *const T = @ptrCast(&a);
+    const c = b.x;
+    _ = c;
+}