tree-optimization/115278 - fix DSE in if-conversion wrt volatiles

The following adds the missing guard for volatile stores to the
embedded DSE in the loop if-conversion pass.

	PR tree-optimization/115278
	* tree-if-conv.cc (ifcvt_local_dce): Do not DSE volatile stores.

	* g++.dg/vect/pr115278.cc: New testcase.