Move lowerConstant to AsmPrinter

This was a static function before, and NVPTX duplicated it
because it wasn't exposed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224354 91177308-0d34-0410-b5e6-96231b3b80d8
NVPTX: Remove duplicate of AsmPrinter::lowerConstant

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224355 91177308-0d34-0410-b5e6-96231b3b80d8