[InstCombine] lshr (sext iM X to iN), N-M --> zext (ashr X, min(N-M, M-1)) to iN

This is a follow-up to https://reviews.llvm.org/D33879 / https://reviews.llvm.org/rL304939 ,
and was discussed in https://reviews.llvm.org/D33338.

We prefer this form because a narrower shift may be cheaper, and we can more easily fold a
zext than a sext.

http://rise4fun.com/Alive/slVe

Name: shz
%s = sext i8 %x to i12
%r = lshr i12 %s, 4
=>
%a = ashr i8 %x, 4
%r = zext i8 %a to i12 

llvm-svn: 305190