range-ops: Calculate the popcount of a singleton.

range-ops: Calculate the popcount of a singleton.

The legacy popcount folding didn't actually fold singleton ranges.
I don't think anyone noticed because there are match.pd patterns that
fold __builtin_popcount using the global nonzero bits set by CCP.

It's good form to handle this, even without CCP's help.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* gimple-range-op.cc (cfn_popcount): Calculate the popcount of a
	singleton.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/popcount6b.c: New test.