[AMDGPU] PHI node cost should not be counted for the size and latency.

  Details: https://reviews.llvm.org/D96805 changed the GCNTTIImpl::getCFInstrCost to return 1 for the PHI nodes
  for the TTI::TCK_CodeSize and TTI::TCK_SizeAndLatency. This is incorrect because the value moves that are the
  result of the PHI lowering are inserted into the basic block predecessors - not into the block itself.
  As a result of this change LoopRotate and LoopUnroll were broken because of the incorrect Loop header and loop
  body size/cost estimation.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D105104