[ARM GlobalISel] Legalize G_FMA

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@322366 91177308-0d34-0410-b5e6-96231b3b80d8