[NVPTX] Fix poorly designed assertion introduced in D120129

NVPTXTargetLowering::getFunctionParamOptimizedAlign, which was introduces in
D120129, contained a poorly designed assertion checking that a function with
internal or private linkage is not a kernel. It relied on invariants that
were not actually guaranteed, and that resulted in compiler crash with some
CUDA versions (see discussion with @jdoerfert in D120129). This patch changes
that assertion and makes it use isKernelFunction which is designed exactly for
such checks. This patch also includes a test with IR that caused compiler crash
before.

Differential Revision: https://reviews.llvm.org/D122562