testsuite: Fix sve/pcs/args_1.c failures [PR116604]

testsuite: Fix sve/pcs/args_1.c failures [PR116604]

This test has been failing since r15-1619-g3b9b8d6cfdf593, which made
IRA prefer a call-clobbered register over a call-preserved register
for mem1 (the second load).  In this particular case, that just
forces the variable p3 to be allocated to a call-preserved register
instead, leading to an extra predicate move from p3 to that register.

However, it was really pot luck that this worked before.  Each argument
is used exactly once, so there isn't an obvious colouring order.
And mem0 and mem1 are passed by indirect reference, so they are not
REG_EQUIV to a stack slot in the way that some memory arguments are.

IIRC, the test was the result of some experimentation, and so I think
the best fix is to rework it to try to make it less sensitive to RA
decisions.  This patch does that by enabling scheduling for the
function and using both memory arguments in the same instruction.
This gets rid of the distracting prologue and epilogue code and
restricts the test to the PCS parts.

gcc/testsuite/
	PR testsuite/116604
	* gcc.target/aarch64/sve/pcs/args_1.c (callee_pred): Enable scheduling
	and use both memory arguments in the same instruction.  Expect no
	prologue and epilogue code.