[NVPTX] Improve lowering of byval args of device functions.

Avoid unnecessary spills of byval arguments of device functions to
local space on SASS level and subsequent pointer conversion to generic
address space that follows. Instead, make a local copy in IR, provide
a way to access arguments directly, and let LLVM optimize the copy away
when possible.

Differential Review: https://reviews.llvm.org/D21421

llvm-svn: 276153