[TTI][AArch64] Add basic vector_reduce_fmaximum/vector_reduce_fminimum costmodelling

This adds some basic handling in TargetTransformInfo to treat
vector_reduce_fminimum/vector_reduce_fmaximum similar to
vector_reduce_fmax/vector_reduce_fmax, getting better costs via
getMinMaxReductionCost.

Differential Revision: https://reviews.llvm.org/D153548