Teach CorrelatedValuePropagation to reduce the width of udiv/urem instructions.

Summary:
If the operands of a udiv/urem can be proved to fit within a smaller
power-of-two-sized type, reduce the width of the udiv/urem.

Reviewers: spatel, sanjoy

Subscribers: llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D44102

llvm-svn: 326898