[AArch64] Mark vecreduce_fminimum and vecreduce_fmaximum as legal

This adds some simple lowering of vecreduce_fminimum and vecreduce_fmaximum to
fminv/fmaxv instructions, in the same way that vecreduce_fmax / vecreduce_fmin
is lowered to fminnmv / fmaxnmv.

Differential Revision: https://reviews.llvm.org/D153246