[LSR] canonicalize Prod*(1<<C) to Prod<<C

Summary:
Because LSR happens at a late stage where mul of a power of 2 is
typically canonicalized to shl, this canonicalization emits code that
can be better CSE'ed.

Test Plan:
Transforms/LoopStrengthReduce/shl.ll shows how this change makes GVN more
powerful. Fixes some existing tests due to this change.

Reviewers: sanjoy, majnemer, atrick

Reviewed By: majnemer, atrick

Subscribers: majnemer, llvm-commits

Differential Revision: http://reviews.llvm.org/D10448

llvm-svn: 240573