[NVPTX] Make the alignment an explicit argument to ldu/ldg

Summary:
Instead of specifying the alignment as metadata which may be destroyed by
transformation passes, make the alignment the second argument to ldu/ldg
intrinsic calls.

Test Plan:
ldu-ldg.ll
ldu-i8.ll
ldu-reg-plus-offset.ll

Reviewers: eliben, meheff, jholewinski

Reviewed By: meheff, jholewinski

Subscribers: jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D5093