Sema: fix is_non_null_ptr handling for runtime-known pointers

We can still often determine a comptime result based on the type, even
if the pointer is runtime-known.

Also, we previously used load -> is non null instead of AIR
`is_non_null_ptr` if the pointer is comptime-known, but that's a bad
heuristic. Instead, we should check for the pointer to be
comptime-known, *and* for the load to be comptime-known, and only in
that case should we call `Sema.analyzeIsNonNull`.

Resolves: #22556

test/behavior/optional.zig

@@ -498,6 +498,20 @@ test "optional of noreturn used with orelse" {
     try expect(val == 123);
 }
 
+test "mutable optional of noreturn" {
+    if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
+
+    var a: ?noreturn = null;
+    if (a) |*ptr| {
+        _ = ptr;
+        @compileError("bad");
+    } else {
+        // this is what we expect to hit
+        return;
+    }
+    @compileError("bad");
+}
+
 test "orelse on C pointer" {
 
     // TODO https://github.com/ziglang/zig/issues/6597