AMDGPU: Mark s_barrier as a high latency instruction

These were marked as WriteSALU, which is low latency.
I'm guessing at the value to use, but it should probably
be considered the highest latency instruction.

I'm not sure this has any actual effect since hasSideEffects
probably is preventing any moving of these.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247060 91177308-0d34-0410-b5e6-96231b3b80d8