Use size_type for operator[].

This matches std::vector and is more efficient as it avoids
truncations.

With this the text segment of opt goes from 19705442 bytes
to 19703930 bytes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221973 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/SmallVector.h

@@ -134,11 +134,11 @@ public:
   /// Return a pointer to the vector's buffer, even if empty().
   const_pointer data() const { return const_pointer(begin()); }
 
-  reference operator[](unsigned idx) {
+  reference operator[](size_type idx) {
     assert(begin() + idx < end());
     return begin()[idx];
   }
-  const_reference operator[](unsigned idx) const {
+  const_reference operator[](size_type idx) const {
     assert(begin() + idx < end());
     return begin()[idx];
   }