aarch64: Fix predication of FP8 FDOT insns [PR120986]

aarch64: Fix predication of FP8 FDOT insns [PR120986]

The predication of the SVE2 FP8 dot product insns was relying on the
architectural dependency:

FEAT_FP8DOT2 => FEAT_FP8DOT4

which was relaxed in GCC as of
r15-7480-g299a8e2dc667e795991bc439d2cad5ea5bd379e2, thus leading to
unrecognisable insn ICEs when compiling a two-way FDOT with just
+fp8dot2.  This patch introduces a new mode iterator which selectively
enables the appropriate mode(s) depending on which of the FP8DOT{2,4}
features are available, and uses it to fix the predication of the
patterns.

gcc/ChangeLog:

	PR target/120986
	* config/aarch64/aarch64-sve2.md (@aarch64_sve_dot<mode>):
	Switch mode iterator from SVE_FULL_HSF to new iterator;
	remove insn predicate as this is now taken care of by conditions
	in the mode iterator.
	(@aarch64_sve_dot_lane<mode>): Likewise.
	* config/aarch64/iterators.md (SVE_FULL_HSF_FP8_FDOT): New.

gcc/testsuite/ChangeLog:

	PR target/120986
	* gcc.target/aarch64/pr120986-1.c: New test.