Interaction between precondition and undef inputs

[nunoplopes]

Logging here since I don't have time today to look into this:
There's an interaction of preconditions taking input registers which can be undef. See example below.
Alive1 didn't have this problem because inputs were never undef.

Pre: WillNotOverflowSignedAdd(%x, %y)
  %r = add i4 %x, %y
=>
  %r = add nsw %x, %y

ERROR: Target is more poisonous than source for i4 %r

Example:
i4 %x = undef
i4 %y = undef
Source value: undef
Target value: poison

With -disable-undef-input it goes through:

----------------------------------------
Pre: WillNotOverflowSignedAdd(%x, %y)
  %r = add i4 %x, %y
=>
  %r = add nsw %x, %y

Done: 1
Optimization is correct!

[nunoplopes]

Fixed here: 38dced3