[NVPTX] Removed always-true predicates in NVPTX.

NVPTX stopped supporting GPUs older than sm_20 (Fermi) quite a while back.
Removal of support of pre-Fermi GPUs made a lot of predicates in the NVPTX
backend pointless as they can't ever be false any more.
It's time to retire them. NFC intended.

Differential Revision: https://reviews.llvm.org/D43843

llvm-svn: 326349