[ARM GlobalISel] Legalize G_FMA

For hard float with VFP4, it is legal. Otherwise, we use libcalls.

This needs a bit of support in the LegalizerHelper for soft float
because we didn't handle G_FMA libcalls yet. The support is trivial, as
the only difference between G_FMA and other libcalls that we already
handle is that it has 3 input operands rather than just 2.

llvm-svn: 322366