[LLVMdev] confusion w.r.t. scalar evolution and nuw

[Sanjoy Das]

> We are permitted to turn 'sub nsw i32 %x, 1' into 'add nsw i32 %x, -1'

nsw also has the same problem:

sub nsw int_min, int_min is 0 but
add nsw int_min, (-int_min) is poison

-- Sanjoy