[NVPTX] Associate a minimum PTX version for each SM architecture

[NVPTX] Associate a minimum PTX version for each SM architecture

When a new SM architecture is introduced, it is only supported by the
current PTX version and later.  Make sure we are using at least the
minimum PTX version for the target architecture.

This also removes support for PTX ISA < 3.2.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233583 91177308-0d34-0410-b5e6-96231b3b80d8