For X86, change load/dec-or-inc/store into dec-or-inc, respectively.

This is a code change to add support for changing instruction sequences of the form:

  load
  inc/dec of 8/16/32/64 bits
  store

into the appropriate X86 inc/dec through memory instruction:

  inc[qlwb] / dec[qlwb]

The checks that were in X86DAGToDAGISel::Select(SDNode *Node)>>ISD::STORE have been extracted to isLoadIncOrDecStore and reworked to use the better
named wrappers for getOperand(unsigned) (e.g. getOffset()) and replaced Chain.getNode() with LoadNode.  The comments have also been expanded.

llvm-svn: 153617