Split the mem-replace codegen test

Apparently in CI it's getting generated in the opposite order, one function per file will make the test pass either way.

tests/codegen/mem-replace-big-type.rs

@@ -0,0 +1,36 @@
+// This test ensures that `mem::replace::<T>` only ever calls `@llvm.memcpy`
+// with `size_of::<T>()` as the size, and never goes through any wrapper that
+// may e.g. multiply `size_of::<T>()` with a variable "count" (which is only
+// known to be `1` after inlining).
+
+// compile-flags: -C no-prepopulate-passes -Zinline-mir=no
+// ignore-debug: the debug assertions get in the way
+
+#![crate_type = "lib"]
+
+#[repr(C, align(8))]
+pub struct Big([u64; 7]);
+pub fn replace_big(dst: &mut Big, src: Big) -> Big {
+    // Before the `read_via_copy` intrinsic, this emitted six `memcpy`s.
+    std::mem::replace(dst, src)
+}
+
+// NOTE(eddyb) the `CHECK-NOT`s ensure that the only calls of `@llvm.memcpy` in
+// the entire output, are the direct calls we want, from `ptr::replace`.
+
+// CHECK-NOT: call void @llvm.memcpy
+
+// For a large type, we expect exactly three `memcpy`s
+// CHECK-LABEL: define internal void @{{.+}}mem{{.+}}replace{{.+}}sret(%Big)
+    // CHECK-NOT: alloca
+    // CHECK: alloca %Big
+    // CHECK-NOT: alloca
+    // CHECK-NOT: call void @llvm.memcpy
+    // CHECK: call void @llvm.memcpy.{{.+}}({{i8\*|ptr}} align 8 %{{.*}}, {{i8\*|ptr}} align 8 %{{.*}}, i{{.*}} 56, i1 false)
+    // CHECK-NOT: call void @llvm.memcpy
+    // CHECK: call void @llvm.memcpy.{{.+}}({{i8\*|ptr}} align 8 %{{.*}}, {{i8\*|ptr}} align 8 %{{.*}}, i{{.*}} 56, i1 false)
+    // CHECK-NOT: call void @llvm.memcpy
+    // CHECK: call void @llvm.memcpy.{{.+}}({{i8\*|ptr}} align 8 %{{.*}}, {{i8\*|ptr}} align 8 %{{.*}}, i{{.*}} 56, i1 false)
+    // CHECK-NOT: call void @llvm.memcpy
+
+// CHECK-NOT: call void @llvm.memcpy

tests/codegen/mem-replace-direct-memcpy.rs

@@ -12,33 +12,11 @@ pub fn replace_byte(dst: &mut u8, src: u8) -> u8 {
     std::mem::replace(dst, src)
 }
 
-#[repr(C, align(8))]
-pub struct Big([u64; 7]);
-pub fn replace_big(dst: &mut Big, src: Big) -> Big {
-    // Before the `read_via_copy` intrinsic, this emitted six `memcpy`s.
-    std::mem::replace(dst, src)
-}
-
 // NOTE(eddyb) the `CHECK-NOT`s ensure that the only calls of `@llvm.memcpy` in
 // the entire output, are the direct calls we want, from `ptr::replace`.
 
 // CHECK-NOT: call void @llvm.memcpy
 
-// For a large type, we expect exactly three `memcpy`s
-// CHECK-LABEL: define internal void @{{.+}}mem{{.+}}replace{{.+}}sret(%Big)
-    // CHECK-NOT: alloca
-    // CHECK: alloca %Big
-    // CHECK-NOT: alloca
-    // CHECK-NOT: call void @llvm.memcpy
-    // CHECK: call void @llvm.memcpy.{{.+}}({{i8\*|ptr}} align 8 %{{.*}}, {{i8\*|ptr}} align 8 %{{.*}}, i{{.*}} 56, i1 false)
-    // CHECK-NOT: call void @llvm.memcpy
-    // CHECK: call void @llvm.memcpy.{{.+}}({{i8\*|ptr}} align 8 %{{.*}}, {{i8\*|ptr}} align 8 %{{.*}}, i{{.*}} 56, i1 false)
-    // CHECK-NOT: call void @llvm.memcpy
-    // CHECK: call void @llvm.memcpy.{{.+}}({{i8\*|ptr}} align 8 %{{.*}}, {{i8\*|ptr}} align 8 %{{.*}}, i{{.*}} 56, i1 false)
-    // CHECK-NOT: call void @llvm.memcpy
-
-// CHECK-NOT: call void @llvm.memcpy
-
 // For a small type, we expect one each of `load`/`store`/`memcpy` instead
 // CHECK-LABEL: define internal noundef i8 @{{.+}}mem{{.+}}replace
     // CHECK-NOT: alloca