[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.

llvm-svn: 233583
diff --git a/llvm/test/CodeGen/NVPTX/sm-version-52.ll b/llvm/test/CodeGen/NVPTX/sm-version-52.ll
index 9b8fc72..18881b2 100644
--- a/llvm/test/CodeGen/NVPTX/sm-version-52.ll
+++ b/llvm/test/CodeGen/NVPTX/sm-version-52.ll
@@ -2,5 +2,6 @@
 ; RUN: llc < %s -march=nvptx64 -mcpu=sm_52 | FileCheck %s
 
 
+; CHECK: .version 4.1
 ; CHECK: .target sm_52