vect: Replace hardcoded inner loop cost factor

vect: Replace hardcoded inner loop cost factor

This patch is to replace the current hardcoded weight factor
50, which is applied by the loop vectorizer to the cost of
statements in an inner loop relative to the loop being
vectorized, with one newly added member inner_loop_cost_factor
in loop vinfo.  It also introduces one parameter
vect-inner-loop-cost-factor whose default value is 50, and
is used to initialize the inner_loop_cost_factor member.

The motivation here is that: if targets want to have one
unique function to gather some information in each add_stmt_cost
call, no matter that it's put before or after the cost tweaking
part for inner loop, it may have the need to adjust (expand or
shrink) the gathered data as the factor.  Now the factor is
hardcoded, it's not easily maintained.

Bootstrapped/regtested on powerpc64le-linux-gnu P9,
x86_64-redhat-linux and aarch64-linux-gnu.

gcc/ChangeLog:

	* doc/invoke.texi (vect-inner-loop-cost-factor): Document new
	parameter.
	* params.opt (vect-inner-loop-cost-factor): New.
	* targhooks.c (default_add_stmt_cost): Replace hardcoded factor
	50 with LOOP_VINFO_INNER_LOOP_COST_FACTOR, include head file
	tree-vectorizer.h and its required ones.
	* config/aarch64/aarch64.c (aarch64_add_stmt_cost): Replace
	hardcoded factor 50 with LOOP_VINFO_INNER_LOOP_COST_FACTOR.
	* config/arm/arm.c (arm_add_stmt_cost): Likewise.
	* config/i386/i386.c (ix86_add_stmt_cost): Likewise.
	* config/rs6000/rs6000.c (rs6000_add_stmt_cost): Likewise.
	* tree-vect-loop.c (vect_compute_single_scalar_iteration_cost):
	Likewise.
	(_loop_vec_info::_loop_vec_info): Init inner_loop_cost_factor.
	* tree-vectorizer.h (_loop_vec_info): Add inner_loop_cost_factor.
	(LOOP_VINFO_INNER_LOOP_COST_FACTOR): New macro.