aarch64: Robustify stack tie handling

aarch64: Robustify stack tie handling

The SVE handling of stack clash protection copied the stack
pointer to X11 before the probe and set up X11 as the CFA
for unwind purposes:

    /* This is done to provide unwinding information for the stack
       adjustments we're about to do, however to prevent the optimizers
       from removing the R11 move and leaving the CFA note (which would be
       very wrong) we tie the old and new stack pointer together.
       The tie will expand to nothing but the optimizers will not touch
       the instruction.  */
    rtx stack_ptr_copy = gen_rtx_REG (Pmode, STACK_CLASH_SVE_CFA_REGNUM);
    emit_move_insn (stack_ptr_copy, stack_pointer_rtx);
    emit_insn (gen_stack_tie (stack_ptr_copy, stack_pointer_rtx));

    /* We want the CFA independent of the stack pointer for the
       duration of the loop.  */
    add_reg_note (insn, REG_CFA_DEF_CFA, stack_ptr_copy);
    RTX_FRAME_RELATED_P (insn) = 1;

-fcprop-registers is now smart enough to realise that X11 = SP,
replace X11 with SP in the stack tie, and delete the instruction
created above.

This patch tries to prevent that by making stack_tie fussy about
the register numbers.  It fixes failures in
gcc.target/aarch64/sve/pcs/stack_clash*.c.

gcc/
	* config/aarch64/aarch64.md (stack_tie): Hard-code the first
	register operand to the stack pointer.  Require the second register
	operand to have the number specified in a separate const_int operand.
	* config/aarch64/aarch64.cc (aarch64_emit_stack_tie): New function.
	(aarch64_allocate_and_probe_stack_space): Use it.
	(aarch64_expand_prologue, aarch64_expand_epilogue): Likewise.
	(aarch64_expand_epilogue): Likewise.