[NVPTX] Only lower sin/cos to approximate instructions if unsafe math is allowed.

Previously we'd always lower @llvm.{sin,cos}.f32 to {sin.cos}.approx.f32
instruction even when unsafe FP math was not allowed.

Clang-generated IR is not affected by this as it uses precise sin/cos
from CUDA's libdevice when unsafe math is disabled.

Differential Revision: https://reviews.llvm.org/D28619

llvm-svn: 291936