[InstCombine] fix and enhance udiv/urem narrowing

There are 3 small independent changes here:

  1. Account for multiple uses in the pattern matching: avoid the transform if it increases the instruction count.
  2. Add a missing fold for the case where the numerator is the constant: http://rise4fun.com/Alive/E2p
  3. Enable all folds for vector types.

There's still one more potential change - use "shouldChangeType()" to keep from transforming to an illegal integer type.

Differential Revision: https://reviews.llvm.org/D36988

llvm-svn: 311726