[NVPTX] Add more surface/texture intrinsics, including CUDA unified texture fetch

[NVPTX] Add more surface/texture intrinsics, including CUDA unified texture fetch

This also uses TSFlags to mark machine instructions that are surface/texture
accesses, as well as the vector width for surface operations.  This is used
to simplify some of the switch statements that need to detect surface/texture
instructions

llvm-svn: 213256