Auto merge of rust-lang#108442 - scottmcm:mir-transmute, r=oli-obk

Add `CastKind::Transmute` to MIR

~~Nothing actually produces it in this commit, so I don't know how to test it, but it also means it shouldn't be possible for it to break anything.~~

Includes lowering `transmute` calls to it, so it's used.

Zulip Conversation: <https://rust-lang.zulipchat.com/#narrow/stream/189540-t-compiler.2Fwg-mir-opt/topic/Good.20first.20isssue/near/321849610>

Author: bors

core/src/intrinsics/mir.rs

@@ -343,6 +343,14 @@ define!(
     /// See [`Field`] for documentation.
     fn Variant<T>(place: T, index: u32) -> ()
 );
+define!(
+    "mir_cast_transmute",
+    /// Emits a `CastKind::Transmute` cast.
+    ///
+    /// Needed to test the UB when `sizeof(T) != sizeof(U)`, which can't be
+    /// generated via the normal `mem::transmute`.
+    fn CastTransmute<T, U>(operand: T) -> U
+);
 define!(
     "mir_make_place",
     #[doc(hidden)]