target/123603 - add --param ix86-vect-compare-costs

target/123603 - add --param ix86-vect-compare-costs

The following allows to switch the x86 target to use the vectorizer
cost comparison mechanic to select between different vector mode
variants of vectorizations.  The default is still to not do this
but this allows an opt-in.

On SPEC CPU 2017 for -Ofast -march=znver4 this shows 2463 out of
39706 vectorized loops changing mode.  In 503 out of 12378 cases
we decided to not use masked epilogs.  Compile-time increases by ~1% overall.
With a quick 1-run there does not seem to be off-noise effects
for INT, this particular optimization and target option combination
and actual hardware to run on.  For FP 549.fotonik3d_r improves by 6%
(confirmed with a 2-run).

This was triggered by PR123190 and PR123603 which have cases where
comparing costs would have resulted in the faster vector size to be
used.  Both were reported for -O2 -march=x86-64-v3 -flto and with PGO.
The PR123603 recorded regression of 548.exchange2_r with these flags
is resolved with the flag (performance improves by 13%).  I don't
have SPEC 2006 on that machine so did not verify the PR123190 433.milc
regression, but that has been improved with the two earlier patches.
The --param has no effect on the testcase in the PR.

I do expect that some of our tricks in the x86 cost model to make
larger vector sizes unprofitable will be obsolete or are
counter-productive with cost comparison turned on.

	PR target/123603
	* config/i386/i386.opt (-param=ix86-vect-compare-costs=): Add.
	* config/i386/i386.cc (ix86_autovectorize_vector_modes): Honor it.
	* doc/invoke.texi (ix86-vect-compare-costs): Document.

	* gcc.dg/vect/costmodel/x86_64/costmodel-pr123603.c: New testcase.