[InstCombine] fold adds of constants separated by sext/zext

This is part of a transform that may be done in the backend:
D13757
...but it should always be beneficial to fold this sooner in IR
for all targets.

https://rise4fun.com/Alive/vaiW

  Name: sext add nsw
  %add = add nsw i8 %i, C0
  %ext = sext i8 %add to i32
  %r = add i32 %ext, C1
  =>
  %s = sext i8 %i to i32
  %r = add i32 %s, sext(C0)+C1

  Name: zext add nuw
  %add = add nuw i8 %i, C0
  %ext = zext i8 %add to i16
  %r = add i16 %ext, C1
  =>
  %s = zext i8 %i to i16
  %r = add i16 %s, zext(C0)+C1

llvm-svn: 355118