Fix DIExpression::ExprOperand::appendToVector

appendToVector used the wrong overload of SmallVector::append, resulting
in it appending the same element to a vector `getSize()` times. This did
not cause a problem when initially committed because appendToVector was
only used to append 1-element operands.

This changes appendToVector to use the correct overload of append().

Testing: ./unittests/IR/IRTests --gtest_filter='*DIExpressionTest*'
llvm-svn: 336466