Does select based on undef result in undef?

[LebedevRI]

Come up in https://reviews.llvm.org/D63060

----------------------------------------
  %sat = usub_sat i8 %a, %b
  %res = add i8 %sat, %b
=>
  %sat = usub_sat i8 %a, %b
  %t0 = icmp ult i8 %a, %b
  %res = select i1 %t0, i8 %b, i8 %a

ERROR: Value mismatch for i8 %res

Example:
i8 %a = undef
i8 %b = #x10 (16)
i8 %sat = undef
i1 %t0 = undef
Source value: undef
Target value: #x02 (2)

So we're selecting based on %t0, which is undef.
The result should be undef, too, right?

CC @nikic.

[LebedevRI]

(moved usub_sat side of the question to #65)

[nunoplopes]

The transformation seems wrong to me, so Alive is correct in complaining. The counterexample is bogus.
Essentially the LHS is not undef. E.g. take a = undef & b = 0x80.

%sat = undef - 0x80 = [0, 0x7F]
%sat + 0x80 = [0x80, 0xFF]

RHS is undef because we get: undef < 0x80 ? 0x80 : undef, which is undef ? 0x80 : undef, which is undef.

[LebedevRI]

@nunoplopes thank you!