Use immediate_backend_type when reading from a const alloc

tests/ui/codegen/const-bool-bitcast.rs

@@ -0,0 +1,15 @@
+// This is a regression test for https://github.com/rust-lang/rust/issues/118047
+// build-pass
+// compile-flags: -Zmir-opt-level=0 -Zmir-enable-passes=+DataflowConstProp
+
+#![crate_type = "lib"]
+
+pub struct State {
+    inner: bool
+}
+
+pub fn make() -> State {
+    State {
+        inner: true
+    }
+}