[DebugInfo] Fix faulty DIExpression::appendToStack assert (#85255)

The appendToStack() function asserts that no DW_OP_stack_value or
DW_OP_LLVM_fragment operations are present in the operations to be
appended. The function did that by iterating over all elements in the
array rather than just the operations, leading it to falsely asserting
on the following input produced by getExt(), since 159 (0x9f) is the
DWARF code for DW_OP_stack_value:

  {dwarf::DW_OP_LLVM_convert, 159, dwarf::DW_ATE_signed}

Fix this by using expr_op iterators.