[CUDA/NVPTX] Improve handling of memcpy for -Os compilations.

We had some instances when LLVM would not inline fixed-count memcpy and ended up
attempting to lower it a a libcall, which would not work on NVPTX as there's no
standard library to call.

The patch relaxes the threshold used for -Os compilation so we're always allowed
to inline memory copy functions.

Differential Revision: https://reviews.llvm.org/D158226