How to gimplify_build a FMA_EXPR since it was removed?

[Laurent Thévenoux <lrnt@xxxxxxxxx>]

Hi,

I've developed some code in gcc/tree-complex.c. I was using the
FMA_EXPR to generate fma operations such as in:

```
rr  = gimplify_build3 (gsi, FMA_EXPR, inner_type, ai, ai, p1r);
```

FMA_EXPR was removed in https://gcc.gnu.org/ml/gcc-patches/2018-05/msg00570.html.

Is there an easy way to gimplify FMAs with the new functions
introduced in the patch below?

Thanks,
Laurent