Hi,

This patch is to add the cost modeling for vector with length,
it mainly follows what we generate for vector with length in
functions vect_set_loop_controls_directly and vect_gen_len
at the worst case.

For Power, the length is expected to be in bits 0-7 (high bits),
we have to model the cost of shifting bits.  To allow other targets
not suffer this, I used one target hook to describe this extra cost,
I'm not sure if it's a correct way.

Bootstrapped/regtested on powerpc64le-linux-gnu (P9) with explicit
param vect-partial-vector-usage=1.

Any comments/suggestions are highly appreciated!

BR,
Kewen
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gcc/ChangeLog:

	* config/rs6000/rs6000.c (TARGET_VECTORIZE_EXTRA_LENGTH_COST): New
	macro.
	* doc/tm.texi: Regenerate.
	* doc/tm.texi.in (TARGET_VECTORIZE_EXTRA_LENGTH_COST): New target hook.
	* target.def (extra_length_cost): Likewise.
	* tree-vect-loop.c (vect_estimate_min_profitable_iters): Add cost
	modeling for vector with length.