[NVPTX] Add support for module-scope inline asm

Since we were explicitly not calling AsmPrinter::doInitialization,
any module-scope inline asm was not being printed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185336 91177308-0d34-0410-b5e6-96231b3b80d8