[DAGCombiner] fold sext into decrement

This is a sibling to rL357178 that I noticed we'd hit if we chose
an alternate transform in D59818.

  %z = zext i8 %x to i32
  %dec = add i32 %z, -1
  %r = sext i32 %dec to i64
  =>
  %z2 = zext i8 %x to i64
  %r = add i64 %z2, -1

https://rise4fun.com/Alive/kPP

The x86 vector diffs show a slight regression, so there's a chance
that we should limit this and the previous transform to scalars.

But given that we allowed vectors before, I'm matching that behavior
here. We should change both transforms together if that's the right
thing to do.

llvm-svn: 357254