arm: remove constraints from *pop_multiple_with_writeback_and_return

This pattern is intended to be used only by the epilogue generation
code and will always use fixed hard registers.  As such, it does not
need any register constraints, which might be misleading if a
post-reload pass wanted to try renumbering various registers.  So
remove the constraints.

Futhermore, to permit this pattern to match when popping just the PC
(which is not a valid register_operand), remove the match on the first
transfer register: pop_multiple_return will validate everything it
needs to.

gcc/ChangeLog:

	* config/arm/arm.md (*pop_multiple_with_writeback_and_return): Remove
	constraints.  Don't validate the first transfer register here.