[NVPTX] run SROA after NVPTXFavorNonGenericAddrSpaces

Summary:
This cleans up most allocas NVPTXLowerKernelArgs emits for byval
parameters.

Test Plan: makes bug21465.ll more stronger to verify no redundant local load/store.

Reviewers: eliben, jholewinski

Reviewed By: eliben, jholewinski

Subscribers: jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D10322

llvm-svn: 239368