[ARM] Use isFMAFasterThanFMulAndFAdd for scalars as well as MVE vectors

This adds extra scalar handling to isFMAFasterThanFMulAndFAdd, allowing
the target independent code to handle more folds in more situations (for
example if the fast math flags are present, but the global
AllowFPOpFusion option isnt). It also splits apart the HasSlowFPVMLx
into HasSlowFPVFMx, to allow VFMA and VMLA to be controlled separately
if needed.

Differential Revision: https://reviews.llvm.org/D72139