Account for memory locations in DIExpression::createFragmentExpression

createFragmentExpression rejects expressions containing certain ops, like
DW_OP_plus, that may cause the expression to compute a value that can't be
split.

Teach createFragmentExpression that the value loaded from an address computed
using those ops is safe to split.

Update a unittest to account for and test this change.

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D136243