AMDGPU: Enhancement on FDIV lowering in AMDGPUCodeGenPrepare

Summary:
  The accuracy limit to use rcp is adjusted to 1.0 ulp from 2.5 ulp.
Also, afn instead of arcp is used to allow inaccurate rcp to be used.

Reviewers:
  arsenm

Differential Revision: https://reviews.llvm.org/D73588