aarch64: Explicitly record probe registers in frame info

The stack frame is currently divided into three areas:

A: the area above the hard frame pointer
B: the SVE saves below the hard frame pointer
C: the outgoing arguments

If the stack frame is allocated in one chunk, the allocation needs a
probe if the frame size is >= guard_size - 1KiB.  In addition, if the
function is not a leaf function, it must probe an address no more than
1KiB above the outgoing SP.  We ensured the second condition by

(1) using single-chunk allocations for non-leaf functions only if
    the link register save slot is within 512 bytes of the bottom
    of the frame; and

(2) using the link register save as a probe (meaning, for instance,
    that it can't be individually shrink wrapped)

If instead the stack is allocated in multiple chunks, then:

* an allocation involving only the outgoing arguments (C above) requires
  a probe if the allocation size is > 1KiB

* any other allocation requires a probe if the allocation size
  is >= guard_size - 1KiB

* second and subsequent allocations require the previous allocation
  to probe at the bottom of the allocated area, regardless of the size
  of that previous allocation

The final point means that, unlike for single allocations,
it can be necessary to have both a non-SVE register probe and
an SVE register probe.  For example:

* allocate A, probe using a non-SVE register save
* allocate B, probe using an SVE register save
* allocate C

The non-SVE register used in this case was again the link register.
It was previously used even if the link register save slot was some
bytes above the bottom of the non-SVE register saves, but an earlier
patch avoided that by putting the link register save slot first.

As a belt-and-braces fix, this patch explicitly records which
probe registers we're using and allows the non-SVE probe to be
whichever register comes first (as for SVE).

The patch also avoids unnecessary probes in sve/pcs/stack_clash_3.c.

gcc/
	* config/aarch64/aarch64.h (aarch64_frame::sve_save_and_probe)
	(aarch64_frame::hard_fp_save_and_probe): New fields.
	* config/aarch64/aarch64.cc (aarch64_layout_frame): Initialize them.
	Rather than asserting that a leaf function saves LR, instead assert
	that a leaf function saves something.
	(aarch64_get_separate_components): Prevent the chosen probe
	registers from being individually shrink-wrapped.
	(aarch64_allocate_and_probe_stack_space): Remove workaround for
	probe registers that aren't at the bottom of the previous allocation.

gcc/testsuite/
	* gcc.target/aarch64/sve/pcs/stack_clash_3.c: Avoid redundant probes.

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