tree-optimization/104880 - move testcase

This renames the testcase to something picked up by the suites regexp.

2022-04-19  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/104880
	* g++.dg/opt/pr104880.cc: Rename to ...
	* g++.dg/opt/pr104880.C: ... this.