tree-optimization/115841 - reduction epilogue placement issue

When emitting the compensation to the vectorized main loop for
a vector reduction value to be re-used in the vectorized epilogue
we fail to place it in the correct block when the main loop is
known to be entered (no loop_vinfo->main_loop_edge) but the
epilogue is not (a loop_vinfo->skip_this_loop_edge).  The code
currently disregards this situation.

With the recent znver4 cost fix I couldn't trigger this situation
with the testcase but I adjusted it so it could eventually trigger
on other targets.

	PR tree-optimization/115841
	* tree-vect-loop.cc (vect_transform_cycle_phi): Correctly
	place the partial vector reduction for the accumulator
	re-use when the main loop cannot be skipped but the
	epilogue can.

	* gcc.dg/vect/pr115841.c: New testcase.

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