[rs6000] Adjust vectorization cost for scalar COND_EXPR

[rs6000] Adjust vectorization cost for scalar COND_EXPR

We found that the vectorization cost modeling on scalar COND_EXPR is a bit off
on rs6000.  One typical case is 548.exchange2_r, -Ofast -mcpu=power9 -mrecip
-fvect-cost-model=unlimited is better than -Ofast -mcpu=power9 -mrecip (the
default is -fvect-cost-model=dynamic) by 1.94%.  Scalar COND_EXPR is expanded
into compare + branch or compare + isel normally, either of them should be
priced more than the simple FXU operation.  This patch is to add additional
vectorization cost onto scalar COND_EXPR on top of builtin_vectorization_cost.
The idea to use additional cost value 2 instead of the others: 1) try various
possible value candidates from 1 to 5, 2 is the best measured on Power9.  2)
from latency view, compare takes 3 cycles and isel takes 2 on Power9, it's
2.5 times of simple FXU instruction which takes cost 1 in the current
modeling, it's close.  3) get fine SPEC2017 ratio on Power8 as well.

gcc/ChangeLog

    * config/rs6000/rs6000.c (adjust_vectorization_cost): New function.
    (rs6000_add_stmt_cost): Call adjust_vectorization_cost and update
    stmt_cost.

From-SVN: r279336