vect: Tweak comparisons with existing epilogue loops

vect: Tweak comparisons with existing epilogue loops

This patch uses a more accurate scalar iteration estimate when
comparing the epilogue of a constant-iteration loop with a candidate
replacement epilogue.

In the testcase, the patch prevents a 1-to-3-element SVE epilogue
from seeming better than a 64-bit Advanced SIMD epilogue.

gcc/
	* tree-vect-loop.c (vect_better_loop_vinfo_p): Detect cases in
	which old_loop_vinfo is an epilogue loop that handles a constant
	number of iterations.

gcc/testsuite/
	* gcc.target/aarch64/sve/cost_model_12.c: New test.