AArch64: Fix signbit mask creation after late combine [PR116229]

The optimization to generate a Di signbit constant by using fneg was relying
on nothing being able to push the constant into the negate.  It's run quite
late for this reason.

However late combine now runs after it and triggers RTL simplification based on
the neg.  When -fno-signed-zeros this ends up dropping the - from the -0.0 and
thus producing incorrect code.

This change adds a new unspec FNEG on DI mode which prevents this simplication.

gcc/ChangeLog:

	PR target/116229
	* config/aarch64/aarch64-simd.md (aarch64_fnegv2di2<vczle><vczbe>): New.
	* config/aarch64/aarch64.cc (aarch64_maybe_generate_simd_constant):
	Update call to gen_aarch64_fnegv2di2.
	* config/aarch64/iterators.md: New UNSPEC_FNEG.

gcc/testsuite/ChangeLog:

	PR target/116229
	* gcc.target/aarch64/pr116229.c: New test.