tree-optimization/107107 - tail-merging VN wrong-code

tree-optimization/107107 - tail-merging VN wrong-code

The following fixes an unintended(?) side-effect of the special
MODIFY_EXPR expression entries we add for tail-merging during VN.
We shouldn't value-number the virtual operand differently here.

	PR tree-optimization/107107
	* tree-ssa-sccvn.c (visit_reference_op_store): Do not
	affect value-numbering when doing the tail merging
	MODIFY_EXPR lookup.

	* gcc.dg/pr107107.c: New testcase.

(cherry picked from commit 85333b9265720fc4e49397301cb16324d2b89aa7)