Rollup merge of #89476 - cjgillot:expn-id, r=petrochenkov

Correct decoding of foreign expansions during incr. comp.

Fixes https://github.com/rust-lang/rust/issues/74946

The original issue was due to a wrong assertion in `expn_hash_to_expn_id`.

The secondary issue was due to a mismatch between the encoding and decoding paths for expansions that are created after the TyCtxt is created.

src/test/incremental/mir-opt.rs

@@ -0,0 +1,11 @@
+// MIR optimizations can create expansions after the TyCtxt has been created.
+// This test verifies that those expansions can be decoded correctly.
+
+// revisions:rpass1 rpass2
+// compile-flags: -Z query-dep-graph -Z mir-opt-level=3
+
+fn main() {
+    if std::env::var("a").is_ok() {
+        println!("b");
+    }
+}