[NVPTX] do not rely on cached subtarget info.

If a module has function references, but no functions
themselves, we may end up never calling runOnMachineFunction
and therefore would never initialize nvptxSubtarget field
which would eventually cause a crash.

Instead of relying on nvptxSubtarget being initialized by
one of the methods, retrieve subtarget info directly.

Differential Revision: https://reviews.llvm.org/D55580

llvm-svn: 348952