AArch64: Define the spaceship optab [PR117013]

AArch64: Define the spaceship optab [PR117013]

This expansion ensures that exactly one comparison is emitted for
spacesip-like sequences on floating-point operands, including when
the result of such sequences are compared against members of
std::<some_ordering>::<some_value>.

For both integer and floating-point types, we optimize for the case
in which the result of a spaceship-like operation is written to a GPR.
The PR highlights this issue for floating-point operands, but we also
make an improvement for integers, preferring:

cmp     w0, w1
cset	w1, gt
csinv	w0, w1, wzr, ge

over:

cmp	w0, w1
mov     w0, 1
csinv   w0, w0, wzr, ge
csel    w0, w0, wzr, ne

to compute:

auto test(int a, int b) { return a <=> b;}

gcc/ChangeLog:
	PR target/117013
	* config/aarch64/aarch64-protos.h (aarch64_expand_fp_spaceship):
	Declare optab expander function for floating-point types.
	* config/aarch64/aarch64.cc (aarch64_expand_fp_spaceship):
	Define optab expansion for floating-point types (new function).
	* config/aarch64/aarch64.md (spaceship<mode>4):
	Add define_expands for spaceship<mode>4 on integer and
	floating-point types.

gcc/testsuite/ChangeLog:
	PR target/117013
	* g++.target/aarch64/spaceship_1.C: New test.
	* g++.target/aarch64/spaceship_2.C: New test.
	* g++.target/aarch64/spaceship_3.C: New test.