Add a MIR test for align_of_val on a slice

tests/mir-opt/instsimplify/align_of_slice.of_val_slice.InstSimplify-after-simplifycfg.diff

@@ -0,0 +1,20 @@
+- // MIR for `of_val_slice` before InstSimplify-after-simplifycfg
++ // MIR for `of_val_slice` after InstSimplify-after-simplifycfg
+  
+  fn of_val_slice(_1: &[T]) -> usize {
+      debug slice => _1;
+      let mut _0: usize;
+      let mut _2: *const [T];
+  
+      bb0: {
+          StorageLive(_2);
+          _2 = &raw const (*_1);
+          _0 = std::intrinsics::align_of_val::<[T]>(move _2) -> [return: bb1, unwind unreachable];
+      }
+  
+      bb1: {
+          StorageDead(_2);
+          return;
+      }
+  }
+  

tests/mir-opt/instsimplify/align_of_slice.rs

@@ -0,0 +1,13 @@
+//@ test-mir-pass: InstSimplify-after-simplifycfg
+//@ needs-unwind
+
+#![crate_type = "lib"]
+#![feature(core_intrinsics)]
+
+// EMIT_MIR align_of_slice.of_val_slice.InstSimplify-after-simplifycfg.diff
+pub fn of_val_slice<T>(slice: &[T]) -> usize {
+    // CHECK-LABEL: fn of_val_slice(_1: &[T])
+    // CHECK: _2 = &raw const (*_1);
+    // CHECK: _0 = std::intrinsics::align_of_val::<[T]>(move _2)
+    unsafe { core::intrinsics::align_of_val(slice) }
+}