[AMDGPU] Use S_DENORM_MODE for gfx10

Summary: During fdiv32 lowering use S_DENORM_MODE to select denorm mode in gfx10.

Reviewers: arsenm, rampitec

Reviewed By: arsenm, rampitec

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65620

llvm-svn: 367882