[DAGCombine] (shl (or x, c1), c2) -> (or (shl x, c2), c1 << c2)

We already have a combine for this pattern when the input to shl is add, so we just need to enable the transformation when the input is or.

Original patch by @tstellar

Differential Revision: https://reviews.llvm.org/D19325

llvm-svn: 313251