+ optimization in tcg64f8086.a_op64_const_reg for OP_ADD and OP_SUB when the
  lowest 48, 32 or 16 bits of the constant are zeros



git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@25705 3ad0048d-3df7-0310-abae-a5850022a9f2