[DebugInfo] Fix for DW_OP_LLVM_fragment in DIExpression::isImplicit()

DIExpression::isImplicit() did not handle DW_OP_LLVM_fragment
correctly. It was scanning the elements in the expression by
iterating from the end. But we do not know the position of
ops unless we iterate from the beginning of the expression,
since DW_OP:s and their operands are stored flat in the expression
list. The old code also assumed that a DW_OP_LLVM_fragment
only occupied one element in the expression list, but it actually
occupies three elements.