Convert the VFP fused multiply-add instructions (VFNMA, VFNMS,
VFMA, VFMS) to decodetree.

Note that in the old decode structure we were implementing
these to honour the VFP vector stride/length. These instructions
were introduced in VFPv4, and in the v7A architecture they
are UNPREDICTABLE if the vector stride or length are non-zero.
In v8A they must UNDEF if stride or length are non-zero, like
all VFP instructions; we choose to UNDEF always.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>