Add --param max-devirt-targets

Add --param max-devirt-targets

Currently we speculatively devirtualize using static analysis (with no profile
feedback) only when there is one possible target found.  With profile feedback
we support multiple targets which seems useful in some scenarios.

This patch adds --param max-devirt-targets which enables devirtualization up to
given number of targets and defaults it to 3.  This happens i.e. in spec2017
omnetpp, though the devirtualizaton is later undone by inliner since it is not
considered useful.  The patch still seems to improve omnetpp by 2% in some
setups.

Other advantage of the patch is that the multi-target devirtualizatoin gets
tested without profile feedback so we more likely notice problems with it.

gcc/ChangeLog:

	* doc/invoke.texi (--param max-devirt-targets) Document.
	* ipa-devirt.cc (ipa_devirt): Implement muti-target
	devirtualization.
	* params.opt (max-devirt-targets): New parameter.

gcc/testsuite/ChangeLog:

	* g++.dg/ipa/devirt-2.C: Update template.
	* g++.dg/ipa/devirt-42.C: Update template.
	* g++.dg/lto/devirt-2_0.C: Update template.