AMDGPU: Fix target options fp32/64-denormals

Fix target options for fp32/64-denormals so that

+fp64-denormals is set if fp64 is supported
-fp32-denormals if fp32 denormals is not supported, or -cl-denorms-are-zero is set
+fp32-denormals if fp32 denormals is supported and -cl-denorms-are-zero is not set

If target feature fp32/64-denormals is explicitly set, they will override default options and options deduced from -cl-denorms-are-zero.

Differential Revision: https://reviews.llvm.org/D24512

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281357 91177308-0d34-0410-b5e6-96231b3b80d8