SLPVectorize: Put horizontal reductions feeding a store under separate flag

Put them under a separate flag for experimentation. They are more likely to
interfere with loop vectorization which happens later in the pass pipeline.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191371 91177308-0d34-0410-b5e6-96231b3b80d8
Set the minimal stack size with msvc when using cmake >= 2.8.11.

This makes sure we get the same behavior with all supported cmake versions. Once
we support only versions >= 2.8.11 we can experiment with other values or just
setting it for some binaries.

Patch by Greg Bedwell.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191372 91177308-0d34-0410-b5e6-96231b3b80d8