[X86] Reduce some 32-bit imuls into lea + shl

Reduce integer multiplication by a constant of the form k*2^c, where k is in {3,5,9} into a lea + shl. Previously it was only done for imulq on 64-bit platforms, but it makes sense for imull and 32-bit as well.

Differential Revision: http://reviews.llvm.org/D7196

llvm-svn: 227308