This helps us form vfnmsub, vfnmadd, and vfmusb from masked VP
intrinsics.
I've used "srcvalue" for the mask parameter in the fneg nodes. We
can't match "V0" because that doesn't sure the mask the is the same.
Instead it matches two different nodes and generates two copies to
V0 of those separate values.
If we don't think srcvalue is ok, we'll need to change to true_mask
or use C++ code to match.
I see a lot of VP intrinsic added, but I'm a little confused about how to use these intrinsics.
I would hazard a guess that the next step is to construct the IR-level VP intrinsic? Is there an initial patch or discussion?