Add CastKind::Transmute to MIR

Updates `interpret`, `codegen_ssa`, and `codegen_cranelift` to consume the new cast instead of the intrinsic.

Includes `CastTransmute` for custom MIR building, to be able to test the extra UB.

compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs

@@ -557,16 +557,6 @@ fn codegen_regular_intrinsic_call<'tcx>(
             fx.bcx.ins().band(ptr, mask);
         }
 
-        sym::transmute => {
-            intrinsic_args!(fx, args => (from); intrinsic);
-
-            if ret.layout().abi.is_uninhabited() {
-                crate::base::codegen_panic(fx, "Transmuting to uninhabited type.", source_info);
-                return;
-            }
-
-            ret.write_cvalue_transmute(fx, from);
-        }
         sym::write_bytes | sym::volatile_set_memory => {
             intrinsic_args!(fx, args => (dst, val, count); intrinsic);
             let val = val.load_scalar(fx);