[NVPTX] Re-enable support for virtual registers in the final output

Now that 3.3 is branched, we are re-enabling virtual registers to help
iron out bugs before the next release. Some of the post-RA passes do
not play well with virtual registers, so we disable them for now. The
needed functionality of the PrologEpilogInserter pass is copied to a
new backend-specific NVPTXPrologEpilog pass.

The test for this commit is not breaking the existing tests.