SCEVValidator: Correctly store 'k * p' as a parameter

We do not only need to understand that 'k * p' is a parameter expression, but
also need to store this expression in the set of parameters. Before this patch
we wrongly stored the two individual parameters %k and %p.

Reported by: Sebastian Pop <spop@codeaurora.org>

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@179485 91177308-0d34-0410-b5e6-96231b3b80d8