[InstCombine] allow (X * C2) << C1 --> X * (C2 << C1) for vectors

This fold already existed for vectors but only when 'C1' was a splat
constant (but 'C2' could be any constant). 

There were no tests for any vector constants, so I'm adding a test
that shows non-splat constants for both operands.  

llvm-svn: 294650