xtensa: Eliminate double MEMW insertions for volatile memory

xtensa: Eliminate double MEMW insertions for volatile memory

This patch makes avoid inserting a MEMW instruction before a load/store
nstruction with volatile memory reference if there is already a MEMW
immediately before it.

gcc/ChangeLog:

	* config/xtensa/xtensa.cc (print_operand):
	When outputting MEMW before the instruction, check if the previous
	instruction is already that.