math-opts: Add dbgcounter for FMA formation

math-opts: Add dbgcounter for FMA formation

This patch is a simple addition of a debug counter to FMA formation in
tree-ssa-math-opts.cc.  Given that issues with FMAs do occasionally
pop up, it seems genuinely useful.

I simply added an if right after the initial checks in
convert_mult_to_fma even though when FMA formation deferring is
active (i.e. when targeting Zen CPUs) this would interact with it (and
at this moment lead to producing all deferred candidates), so when
using the dbg counter to find a harmful set of FMAs, it is probably
best to also set param_avoid_fma_max_bits to zero.  I could not find a
better place which would not also make the code unnecessarily more
complicated.

gcc/ChangeLog:

2023-09-06  Martin Jambor  <mjambor@suse.cz>

	* dbgcnt.def (form_fma): New.
	* tree-ssa-math-opts.cc: Include dbgcnt.h.
	(convert_mult_to_fma): Bail out if the debug counter say so.