tree-optimization/102318 - reduction epilogue re-use

This refines the fix for PR102226 to do the mode conversion
from V2DI to VNx2DI separately from the sign-conversion, retaining
the signedness of the saved accumulator as before the original fix.

2021-09-15  Richard Biener <rguenther@suse.de>

	PR tree-optimization/102318
	* tree-vect-loop.c (vect_transform_cycle_phi): Revert
	previous change and do the mode conversion separately from
	the sign conversion.

	* gcc.dg/vect/pr102318.c: New testcase.