tree-optimization/107767 - not profitable switch conversion

tree-optimization/107767 - not profitable switch conversion

When the CFG has not merged equal PHI defs in a switch stmt the
cost model from switch conversion gets off and we prefer a
jump table over branches.  The following fixes that by recording
cases that will be merged later and more appropriately counting
unique values.

	PR tree-optimization/107767
	* tree-cfgcleanup.cc (phi_alternatives_equal): Export.
	* tree-cfgcleanup.h (phi_alternatives_equal): Declare.
	* tree-switch-conversion.cc (switch_conversion::collect):
	Count unique non-default targets accounting for later
	merging opportunities.

	* gcc.dg/tree-ssa/pr107767.c: New testcase.