[AArch64] FMINNAN/FMAXNAN on f16 is not legal.

Spotted by Ahmed - in r244594 I inadvertently marked f16 min/max as legal.

I've reverted it here, and marked min/max on scalar f16's as promote. I've also added a testcase. The test just checks that the compiler doesn't fall over - it doesn't create fmin nodes for f16 yet.

llvm-svn: 245035