[amdgpu][lds] Use the same isKernel predicate consistently

isKernelCC != isKernel(F->getCallingConv())
There's a test case (lower-kernel-lds.ll) that explicitly skips amdgpu_ps
so this change picks the isKernel predicate that continues to skip that
calling convention.

isKernel returns true for AMDGPU_KERNEL and SPIR_KERNEL. isKernelCC also
returns true for other calling conventions.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D136599