[DAGCombiner] Transform (zext (select c, load1, load2)) -> (select c, zextload1, zextload2)

If extload is legal, following transform
    (zext (select c, load1, load2)) -> (select c, zextload1, zextload2)
can save one ext instruction.

Differential Revision: https://reviews.llvm.org/D95086