Fix an allocation regression due to the recent SegmentedList change c…

src/Compilers/Core/CodeAnalysisTest/Collections/List/SegmentedList.Generic.Tests.Capacity.cs

@@ -140,5 +140,18 @@ public void EnsureCapacity_MatchesSizeWithLargeCapacityRequest(int segmentCount)
 
             Assert.Equal(expectedCapacity, list.Capacity);
         }
+
+        [Fact]
+        public void EnsureCapacity_InitialCapacitySlightlyMoreThanHalfSegmentSizeGrowsToFullSegmentSize()
+        {
+            var elementCount = SegmentedArray<T>.TestAccessor.SegmentSize / 2 + 1;
+            var list = new SegmentedList<T>(elementCount);
+
+            Assert.Equal(elementCount, list.Capacity);
+
+            list.EnsureCapacity(elementCount + 1);
+
+            Assert.Equal(SegmentedArray<T>.TestAccessor.SegmentSize, list.Capacity);
+        }
     }
 }

src/Dependencies/Collections/SegmentedList`1.cs

@@ -520,6 +520,11 @@ internal void Grow(int capacity)
                 // should be DefaultCapacity. Otherwise, the new capacity should be double the current array size.
                 newCapacity = _items.Length == 0 ? DefaultCapacity : _items.Length * 2;
             }
+            else if (_items.Length < SegmentedArrayHelper.GetSegmentSize<T>())
+            {
+                // There is only a single segment that is over half full. Increase it to a full segment.
+                newCapacity = SegmentedArrayHelper.GetSegmentSize<T>();
+            }
             else
             {
                 // If the last segment is fully sized, increase the number of segments by the desired growth rate