swap ops in reassoc to reduce cross backedge FMA

swap ops in reassoc to reduce cross backedge FMA

Previously for ops.length >= 3, when FMA is present, we don't
rank the operands so that more FMAs can be preserved. But this
brings more FMAs with loop dependency, which lead to worse
performance on some targets.

Rank the oprands (set width=2) when:
1. avoid_fma_max_bits is set.
2. And loop dependent FMA sequence is found.

In this way, we don't have to discard all the FMA candidates
in the bad shaped sequence in widening_mul, instead we can keep
fewer FMAs without loop dependency.

With this patch, there's about 2% improvement in 510.parest_r
1-copy run on ampere1 (with "-Ofast -mcpu=ampere1 -flto
--param avoid-fma-max-bits=512").

PR tree-optimization/110279

gcc/ChangeLog:

	* tree-ssa-reassoc.cc (get_reassociation_width): check
	for loop dependent FMAs.
	(reassociate_bb): For 3 ops, refine the condition to call
	swap_ops_for_binary_stmt.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr110279-1.c: New test.