vect: Don't consider branch costs if no peeled iterations

vect: Don't consider branch costs if no peeled iterations

Currently vectorizer cost modeling counts branch taken costs for
prologue and epilogue if the number of iterations is unknown.
But it isn't sensible if there are no peeled iterations.

This patch is to guard them under peel_iters_prologue > 0 or
peel_iters_epilogue > 0.

Bootstrapped/regtested on powerpc64le-linux-gnu and aarch64-linux-gnu.

gcc/ChangeLog:

	* tree-vect-loop.c (vect_get_known_peeling_cost): Don't consider branch
	taken costs for prologue and epilogue if they don't exist.
	(vect_estimate_min_profitable_iters): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/sve/cost_model_2.c: Adjust due to cost model
	change.