[ADT] Add SmallVector::pop_back_n

Adds a method called pop_back_n to SmallVector.
This is more readable and less error prone than the alternatives of using
```lang=c++
Vector.resize(Vector.size() - N);
Vector.erase(Vector.end() - N, Vector.end());
for (unsigned I = 0;I<N;++I) Vector.pop_back();
```

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D90576

2 files changed, 7 insertions, 2 deletions

diff --git a/llvm/include/llvm/ADT/SmallVector.h b/llvm/include/llvm/ADT/SmallVector.h
index bb008af25f96..e042497473db 100644
--- a/llvm/include/llvm/ADT/SmallVector.h
+++ b/llvm/include/llvm/ADT/SmallVector.h
@@ -424,6 +424,12 @@ public:
       this->grow(N);
   }
 
+  void pop_back_n(size_type NumItems) {
+    assert(this->size() >= NumItems);
+    this->destroy_range(this->end() - NumItems, this->end());
+    this->set_size(this->size() - NumItems);
+  }
+
   LLVM_NODISCARD T pop_back_val() {
     T Result = ::std::move(this->back());
     this->pop_back();
diff --git a/llvm/unittests/ADT/SmallVectorTest.cpp b/llvm/unittests/ADT/SmallVectorTest.cpp
index dbe404869e2c..162716abe9ff 100644
--- a/llvm/unittests/ADT/SmallVectorTest.cpp
+++ b/llvm/unittests/ADT/SmallVectorTest.cpp
@@ -261,8 +261,7 @@ TYPED_TEST(SmallVectorTest, PushPopTest) {
   this->assertValuesInOrder(this->theVector, 2u, 2, 1);
 
   // Pop remaining elements
-  this->theVector.pop_back();
-  this->theVector.pop_back();
+  this->theVector.pop_back_n(2);
   this->assertEmpty(this->theVector);
 
   // Check number of constructor calls. Should be 2 for each list element,