[InstCombine] fold add(zext(xor X, C), C) --> sext X when C is INT_MIN in the source...

[InstCombine] fold add(zext(xor X, C), C) --> sext X when C is INT_MIN in the source type

The pattern may look more obviously like a sext if written as:

  define i32 @g(i16 %x) {
    %zext = zext i16 %x to i32
    %xor = xor i32 %zext, 32768
    %add = add i32 %xor, -32768
    ret i32 %add
  }

We already have that fold in visitAdd().

Differential Revision: https://reviews.llvm.org/D22477

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@276035 91177308-0d34-0410-b5e6-96231b3b80d8