[InstCombine] shrink type of sdiv if dividend is sexted and constant divisor...

[InstCombine] shrink type of sdiv if dividend is sexted and constant divisor is small enough (PR28153)

This should fix PR28153:
https://llvm.org/bugs/show_bug.cgi?id=28153

Differential Revision: http://reviews.llvm.org/D21769

llvm-svn: 273951