target/108738 - limit STV chain discovery

target/108738 - limit STV chain discovery

The following puts a hard limit on the inherently quadratic STV chain
discovery.  Without a limit for the compiler.i testcase in PR26854
we see at -O2

 machine dep reorg                  : 574.45 ( 53%)

with release checking while with the proposed limit it's

 machine dep reorg                  :   2.86 (  1%)

	PR target/108738
	* config/i386/i386.opt (--param x86-stv-max-visits): New param.
	* doc/invoke.texi (--param x86-stv-max-visits): Document it.
	* config/i386/i386-features.h (scalar_chain::max_visits): New.
	(scalar_chain::build): Add bitmap parameter, return boolean.
	(scalar_chain::add_insn): Likewise.
	(scalar_chain::analyze_register_chain): Likewise.
	* config/i386/i386-features.cc (scalar_chain::scalar_chain):
	Initialize max_visits.
	(scalar_chain::analyze_register_chain): When we exhaust
	max_visits, abort.  Also abort when running into any
	disallowed insn.
	(scalar_chain::add_insn): Propagate abort.
	(scalar_chain::build): Likewise.  When aborting amend
	the set of disallowed insn with the insns set.
	(convert_scalars_to_vector): Adjust.  Do not convert aborted
	chains.