[InstCombine] Fix wrong folding of constant comparisons involving ashr and negative values.

This patch fixes a bug in method InstCombiner::FoldCmpCstShrCst where we
wrongly computed the distance between the highest bits set of two negative
values.

This fixes PR21222.

Differential Revision: http://reviews.llvm.org/D5700


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219406 91177308-0d34-0410-b5e6-96231b3b80d8