tree-optimization/105012 - fix ICE from local DSE of if-conversion

The following guards dse_classify_store with the same condition as
the DSE pass does - availability of a virtual definition.  For
the PR we run into the fortran frontend generating a clobber for
a FUNCTION_DECL lhs which is ignored by the operand scanner and has
no virtual operands assigned.  Apart from fixing the frontend the
following fixes the ICE by adjusting if-conversion.

2022-03-22  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/105012
	* tree-if-conv.cc (ifcvt_local_dce): Only call
	dse_classify_store when we have a VDEF.