[SeparateConstOffsetFromGEP] inbounds zext => sext for better splitting

For each array index that is in the form of zext(a), convert it to sext(a)
if we can prove zext(a) <= max signed value of typeof(a). The conversion
helps to split zext(x + y) into sext(x) + sext(y).

Reviewed in http://reviews.llvm.org/D4060



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210444 91177308-0d34-0410-b5e6-96231b3b80d8