Reassociation destroys nsw/nuw flags from BinOps that are changed. But if the
expression at the end of a tree that was altered, but didn't change itself,
the flags do not need to be removed. For example, if %a, %b and %c are
reassociated in
%x = add nsw i32 %a, %c
%y = add nsw i32 %x, %b
%z = add nsw i32 %y, %d
The value of %y and so add %y %d remains the same, and %z needn't drop the nsw
flags.
https://alive2.llvm.org/ce/z/_juAiV
Differential Revision: https://reviews.llvm.org/D154289
db32d11a38