[DAGCombiner] Fold together mul and shl when both are by a constant

[DAGCombiner] Fold together mul and shl when both are by a constant

This is intended to improve code generation for GEPs, as the index value is
shifted by the element size and in GEPs of multi-dimensional arrays the index
of higher dimensions is multiplied by the lower dimension size.

Differential Revision: http://reviews.llvm.org/D12197

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245689 91177308-0d34-0410-b5e6-96231b3b80d8