Strengthen the repeat-trusted-len test

Simply checking for the presence of `llvm.memset` is too brittle because
this instrinsic can be used for seemingly trivial operations, such as
zero-initializing a `RawVec`.

src/test/codegen/repeat-trusted-len.rs

@@ -15,9 +15,14 @@
 
 use std::iter;
 
+// CHECK: @helper([[USIZE:i[0-9]+]] %arg0)
+#[no_mangle]
+pub fn helper(_: usize) {
+}
+
 // CHECK-LABEL: @repeat_take_collect
 #[no_mangle]
 pub fn repeat_take_collect() -> Vec<u8> {
-// CHECK: call void @llvm.memset.p0i8
+// CHECK: call void @llvm.memset.p0i8.[[USIZE]](i8* {{(nonnull )?}}%{{[0-9]+}}, i8 42, [[USIZE]] 100000, i32 1, i1 false)
     iter::repeat(42).take(100000).collect()
 }