[01/46] Move special cases out of get_initial_def_for_reduction

[01/46] Move special cases out of get_initial_def_for_reduction

This minor clean-up avoids repeating the test for double reductions
and also moves the vect_get_vec_def_for_operand call to the same
function as the corresponding vect_get_vec_def_for_stmt_copy.

2018-07-31  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-loop.c (get_initial_def_for_reduction): Move special
	cases for nested loops from here to ...
	(vect_create_epilog_for_reduction): ...here.  Only call
	vect_is_simple_use for inner-loop reductions.

From-SVN: r263116