[DAGCombiner] fold sext into negation

As noted in D59818:
  %z = zext i8 %x to i32
  %neg = sub i32 0, %z
  %r = sext i32 %neg to i64
  =>
  %z2 = zext i8 %x to i64
  %r = sub i64 0, %z2

https://rise4fun.com/Alive/KzSR

llvm-svn: 357178