[SLPVectorizer] Ensure dominated reduction values.

When considering incoming values as part of a reduction phi, ensure the
incoming value is dominated by said phi.

Failing to ensure this property causes miscompiles.

Fixes PR25787.

Many thanks to Mattias Eriksson for reporting, reducing and analyzing the
problem for me.

Differential Revision: http://reviews.llvm.org/D15580

llvm-svn: 255792