[InstCombine] reassociate logic ops with constants separated by a zext

This is a partial implementation of a general fold for associative+commutative operators:
(op (cast (op X, C2)), C1) --> (cast (op X, op (C1, C2)))
(op (cast (op X, C2)), C1) --> (op (cast X), op (C1, C2))

There are 7 associative operators and 13 cast types, so this could potentially go a lot further.

Differential Revision: https://reviews.llvm.org/D22421

llvm-svn: 275684