[AMDGPU] Fix regression in AMDGPULibCalls allowing native for doubles

Under -cl-fast-relaxed-math we could use native_sqrt, but f64 was
allowed to produce HSAIL's nsqrt instruction. HSAIL is not here
and we stick with non-existing native_sqrt(double) as a result.

Add check for f64 to not return native functions and also remove
handling of f64 case for fold_sqrt.

Differential Revision: https://reviews.llvm.org/D37223

llvm-svn: 311900