Account for the cost of generating loop masks

Account for the cost of generating loop masks

We didn't take the cost of generating loop masks into account, and so
tended to underestimate the cost of loops that need multiple masks.

2019-11-13  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-loop.c (vect_estimate_min_profitable_iters): Include
	the cost of generating loop masks.

gcc/testsuite/
	* gcc.target/aarch64/sve/mask_struct_store_3.c: Add
	-fno-vect-cost-model.
	* gcc.target/aarch64/sve/mask_struct_store_3_run.c: Likewise.
	* gcc.target/aarch64/sve/peel_ind_2.c: Likewise.
	* gcc.target/aarch64/sve/peel_ind_2_run.c: Likewise.
	* gcc.target/aarch64/sve/peel_ind_3.c: Likewise.
	* gcc.target/aarch64/sve/peel_ind_3_run.c: Likewise.

From-SVN: r278125