[InstCombine] sub X, sext(bool Y) -> add X, zext(bool Y)

Prefer add/zext because they are better supported in terms of value-tracking.

Note that the backend should be prepared for this IR canonicalization 
(including vector types) after:
https://reviews.llvm.org/rL284015

Differential Revision: https://reviews.llvm.org/D25135

llvm-svn: 284241