rs6000: Remove redundant MEM_P predicate usage

The quad_memory_operand and vsx_quad_dform_memory_operand predicates contain
a (match_code "mem") test, making their MEM_P usage redundant.  Remove them.

2023-07-10  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	* config/rs6000/predicates.md (quad_memory_operand): Remove redundant
	MEM_P usage.
	(vsx_quad_dform_memory_operand): Likewise.