[SelectionDAG] Get rid of bool parameters in SelectionDAG::getLoad, getStore, and friends.

Summary:
Instead, we take a single flags arg (a bitset).

Also add a default 0 alignment, and change the order of arguments so the
alignment comes before the flags.

This greatly simplifies many callsites, and fixes a bug in
AMDGPUISelLowering, wherein the order of the args to getLoad was
inverted.  It also greatly simplifies the process of adding another flag
to getLoad.

Reviewers: chandlerc, tstellarAMD

Subscribers: jholewinski, arsenm, jyknight, dsanders, nemanjai, llvm-commits

Differential Revision: http://reviews.llvm.org/D22249

llvm-svn: 275592