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