InstCombine: Don't fold (X <<s log(INT_MIN)) /s INT_MIN to X

Consider the case where X is 2.  (2 <<s 31)/s-2147483648 is zero but we
would fold to X.  Note that this is valid when we are in the unsigned
domain because we require NUW: 2 <<u 31 results in poison.

This fixes PR21245.

llvm-svn: 219568