[NVPTX] Make tensor load/store intrinsics overloaded.

This way we can support address-space specific variants without explicitly
encoding the space in the name of the intrinsic. Less intrinsics to deal with ->
less boilerplate.

Added a bit of tablegen magic to match/replace an intrinsics with a pointer
argument in particular address space with the space-specific instruction
variant.

Updated tests to use non-default address spaces.

Differential Revision: https://reviews.llvm.org/D43268

llvm-svn: 328006