Add a mir-opt test for an unneeded drop_in_place

Author: scottmcm

tests/mir-opt/remove_unneeded_drop_in_place.rs

@@ -0,0 +1,13 @@
+//@ test-mir-pass: RemoveUnneededDrops
+//@ needs-unwind
+
+// EMIT_MIR remove_unneeded_drop_in_place.slice_in_place.RemoveUnneededDrops.diff
+unsafe fn slice_in_place(ptr: *mut [char]) {
+    std::ptr::drop_in_place(ptr)
+}
+
+fn main() {
+    // CHECK-LABEL: fn main(
+    let mut a = ['o', 'k'];
+    unsafe { slice_in_place(&raw mut a) };
+}

tests/mir-opt/remove_unneeded_drop_in_place.slice_in_place.RemoveUnneededDrops.diff

@@ -0,0 +1,20 @@
+- // MIR for `slice_in_place` before RemoveUnneededDrops
++ // MIR for `slice_in_place` after RemoveUnneededDrops
+  
+  fn slice_in_place(_1: *mut [char]) -> () {
+      debug ptr => _1;
+      let mut _0: ();
+      let mut _2: *mut [char];
+  
+      bb0: {
+          StorageLive(_2);
+          _2 = copy _1;
+          _0 = drop_in_place::<[char]>(move _2) -> [return: bb1, unwind continue];
+      }
+  
+      bb1: {
+          StorageDead(_2);
+          return;
+      }
+  }
+