[NVPTX] Lower instructions that expand into libcalls.

The change is an effort to split and refactor abandoned
D34708 into smaller parts.

Here the behaviour of unsupported instructions is changed
to match the behaviour of explicit intrinsics calls.
Currently LLVM crashes with:
> Assertion getInstruction() && "Not a call or invoke instruction!" failed.

With this patch LLVM produces a more sensible error message:
> Cannot select: ... i32 = ExternalSymbol'__foobar'

Author: Denys Zariaiev <denys.zariaiev@gmail.com>

Differential Revision: https://reviews.llvm.org/D55145

llvm-svn: 349213