[AArch64][CodeGen] Fix crash when fptrunc returns fp16 with +nofp attr (#81724)

When performing lowering of the fptrunc opcode returning fp16 with the
+nofp flag enabled we could trigger a compiler crash. This is because we
had no custom lowering implemented. This patch 
the case in which we need to promote an fp16 return type
for fptrunc when the +nofp attr is enabled.