Vect: use a small step to calculate induction for the unrolled loop (PR tree-optimiza...

Vect: use a small step to calculate induction for the unrolled loop (PR tree-optimization/110449)

If a loop is unrolled by n times during vectoriation, two steps are used to
calculate the induction variable:
  - The small step for the unrolled ith-copy: vec_1 = vec_iv + (VF/n * Step)
  - The large step for the whole loop: vec_loop = vec_iv + (VF * Step)

This patch calculates an extra vec_n to replace vec_loop:
  vec_n = vec_prev + (VF/n * S) = vec_iv + (VF/n * S) * n = vec_loop.

So that we can save the large step register and related operations.

gcc/ChangeLog:

	PR tree-optimization/110449
	* tree-vect-loop.cc (vectorizable_induction): use vec_n to replace
	vec_loop for the unrolled loop.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/pr110449.c: New testcase.