[InstCombine] move add after smin/smax

Follow-up to rL355221.
This isn't specifically called for within PR14613,
but we'll get there eventually if it's not already
requested in some other bug report.

https://rise4fun.com/Alive/5b0

  Name: smax
  Pre: WillNotOverflowSignedSub(C1,C0)
  %a = add nsw i8 %x, C0
  %cond = icmp sgt i8 %a, C1
  %r = select i1 %cond, i8 %a, i8 C1
  =>
  %c2 = icmp sgt i8 %x, C1-C0
  %u2 = select i1 %c2, i8 %x, i8 C1-C0
  %r = add nsw i8 %u2, C0

  Name: smin
  Pre: WillNotOverflowSignedSub(C1,C0)
  %a = add nsw i32 %x, C0
  %cond = icmp slt i32 %a, C1
  %r = select i1 %cond, i32 %a, i32 C1
  =>
  %c2 = icmp slt i32 %x, C1-C0
  %u2 = select i1 %c2, i32 %x, i32 C1-C0
  %r = add nsw i32 %u2, C0

llvm-svn: 355272