Enable late-vectorization by default.

Enable late-vectorization by default.
This patch changes the default setting for the LateVectorization flag that controls where the loop-vectorizer is ran.

Perf gains:
SingleSource/Benchmarks/Shootout/matrix -37.33%
MultiSource/Benchmarks/PAQ8p/paq8p  -22.83%
SingleSource/Benchmarks/Linpack/linpack-pc  -16.22%
SingleSource/Benchmarks/Shootout-C++/ary3 -15.16%
MultiSource/Benchmarks/TSVC/NodeSplitting-flt/NodeSplitting-flt -10.34%
MultiSource/Benchmarks/TSVC/NodeSplitting-dbl/NodeSplitting-dbl -7.12%

Regressions:
SingleSource/Benchmarks/Misc/lowercase  15.10%
MultiSource/Benchmarks/TSVC/Equivalencing-flt/Equivalencing-flt 13.18%
SingleSource/Benchmarks/Shootout-C++/matrix 8.27%
SingleSource/Benchmarks/CoyoteBench/lpbench 7.30%

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189858 91177308-0d34-0410-b5e6-96231b3b80d8