fix backend intrinsic calls in cg_ssa

Author: WaffleLapkin

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -982,6 +982,14 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 
                                 return if let Some(target) = target {
                                     helper.funclet_br(self, bx, target, mergeable_succ)
+                                } else if kind == CallKind::Tail {
+                                    let res = bx.load(
+                                        bx.backend_type(result.layout),
+                                        result.val.llval,
+                                        result.val.align,
+                                    );
+                                    bx.ret(res);
+                                    MergingSucc::False
                                 } else {
                                     bx.unreachable();
                                     MergingSucc::False

tests/ui/explicit-tail-calls/intrinsics.rs

@@ -7,7 +7,12 @@ fn trans((): ()) {
     unsafe { become std::mem::transmute(()) }
 }
 
+fn cats(x: u64) -> u32 {
+    become std::intrinsics::ctlz(x)
+}
+
 fn main() {
     trans(());
+    assert_eq!(cats(17), 59);
 }