[NFC][CodeGen] Remove rarely used DL variable from SelectionDAGBuilder

There is a pointer to the DataLayout in SelectionDAGBuilder called
'DL' that is hardly ever used. In most cases the code seems to just
use `DAG.getDataLayout()` instead. Given that DL is also often used
as a shadowed variable for the debug location it seems sensible to
just kill off the few remaining uses and be consistent with the rest
of the code.

Differential Revision: https://reviews.llvm.org/D114451