This patch is to fix the regressions found in SPEC2017 fprate cases
 on aarch64.

1. Reused code in pass widening_mul to check for nested FMA chains
 (those connected by MULT_EXPRs), since re-writing to parallel
 generates worse codes.

2. Avoid re-arrange to produce less FMA chains that can be slow.

Tested on ampere1 and neoverse-n1, this fixed the regressions in
508.namd_r and 510.parest_r 1 copy run. While I'm still collecting data
on x86 machines we have, I'd like to know what do you think of this.

(Previously I tried to improve things with FMA by adding a widening_mul
pass before reassoc2 for it's easier to recognize different patterns
of FMA chains and decide whether to split them. But I suppose handling
them all in reassoc pass is more efficient.)

Thanks,
Di Zhao

---
gcc/ChangeLog:

        * tree-ssa-math-opts.cc (convert_mult_to_fma_1): Add new parameter.
        Support new mode that merely do the checking. 
        (struct fma_transformation_info): Moved to header.
        (class fma_deferring_state): Moved to header.
        (convert_mult_to_fma): Add new parameter.
        * tree-ssa-math-opts.h (struct fma_transformation_info):
        (class fma_deferring_state): Moved from .cc.
        (convert_mult_to_fma): Add function decl.
        * tree-ssa-reassoc.cc (rewrite_expr_tree_parallel):
        (rank_ops_for_fma): Return -1 if nested FMAs are found.
        (reassociate_bb): Avoid rewriting to parallel if nested FMAs are found.