ac: set no-signed-zeros-fp-math when RADV_DEBUG="unsafemath" is used

This is an optimisation that is recommended by Matt Arsenault,
and used by RadeonSI, but it's not compatible with Vulkan.

Note that AC_FLOAT_MODE_UNSAFE_FP_MATH includes the no signed
zeros flag in LLVM.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>

src/amd/common/ac_nir_to_llvm.c

@@ -352,6 +352,9 @@ create_llvm_function(LLVMContextRef ctx, LLVMModuleRef module,
 		LLVMAddTargetDependentFunctionAttr(main_function,
 						   "unsafe-fp-math",
 						   "true");
+		LLVMAddTargetDependentFunctionAttr(main_function,
+					   "no-signed-zeros-fp-math",
+					   "true");
 	}
 	return main_function;
 }