At opt-level=0, apply only ABI-affecting attributes to functions

This should provide a small perf improvement for debug builds,
and should more than cancel out the regression from adding noundef,
which was only significant in debug builds.

src/test/codegen/function-arguments-noopt.rs

@@ -0,0 +1,63 @@
+// compile-flags: -C opt-level=0 -C no-prepopulate-passes
+
+// This test checks that arguments/returns in opt-level=0 builds,
+// while lacking attributes used for optimization, still have ABI-affecting attributes.
+
+#![crate_type = "lib"]
+#![feature(rustc_attrs)]
+
+pub struct S {
+  _field: [i32; 8],
+}
+
+// CHECK: define zeroext i1 @boolean(i1 zeroext %x)
+#[no_mangle]
+pub fn boolean(x: bool) -> bool {
+  x
+}
+
+// CHECK-LABEL: @boolean_call
+#[no_mangle]
+pub fn boolean_call(x: bool, f: fn(bool) -> bool) -> bool {
+// CHECK: call zeroext i1 %f(i1 zeroext %x)
+  f(x)
+}
+
+// CHECK: define i32* @borrow(i32* %x)
+#[no_mangle]
+pub fn borrow(x: &i32) -> &i32 {
+  x
+}
+
+// CHECK-LABEL: @borrow_call
+#[no_mangle]
+pub fn borrow_call(x: &i32, f: fn(&i32) -> &i32) -> &i32 {
+  // CHECK: call i32* %f(i32* %x)
+  f(x)
+}
+
+// CHECK: define void @struct_(%S* sret(%S){{( %0)?}}, %S* %x)
+#[no_mangle]
+pub fn struct_(x: S) -> S {
+  x
+}
+
+// CHECK-LABEL: @struct_call
+#[no_mangle]
+pub fn struct_call(x: S, f: fn(S) -> S) -> S {
+  // CHECK: call void %f(%S* sret(%S){{( %0)?}}, %S* %{{.+}})
+  f(x)
+}
+
+// CHECK: define { i8, i8 } @enum_(i1 zeroext %x.0, i8 %x.1)
+#[no_mangle]
+pub fn enum_(x: Option<u8>) -> Option<u8> {
+  x
+}
+
+// CHECK-LABEL: @enum_call
+#[no_mangle]
+pub fn enum_call(x: Option<u8>, f: fn(Option<u8>) -> Option<u8>) -> Option<u8> {
+  // CHECK: call { i8, i8 } %f(i1 zeroext %x.0, i8 %x.1)
+  f(x)
+}