[LICM] Mul association incorrectly combines no-wrap flags

[aleks-tmb]

After the #67736 LICM turns the following IR

define i32 @test() {
entry:
  br label %loop

loop:
  %iv = phi i32 [ 1, %entry ], [ %iv.next, %loop ]
  %iv.next = add nuw nsw i32 %iv, 1
  %mul0 = mul nuw nsw i32 30000, %iv
  %mul1 = mul nuw i32 %mul0, 30000
  %cmp = icmp slt i32 %mul1, 1
  br i1 %cmp, label %exit, label %loop

exit:
  ret i32 0
}

into the

define i32 @test() {
entry:
  br label %loop

loop:                                             ; preds = %loop, %entry
  %iv = phi i32 [ 1, %entry ], [ %iv.next, %loop ]
  %iv.next = add nuw nsw i32 %iv, 1
  %mul0 = mul nuw nsw i32 900000000, %iv
  %cmp = icmp slt i32 %mul0, 1
  br i1 %cmp, label %exit, label %loop

exit:                                             ; preds = %loop
  ret i32 0
}

https://godbolt.org/z/G5zvEzYEv

The resulting mul instruction has 2 flags nuw nsw, while in the source code it has nuw only. That transformation seems to be incorrect.

If we run instcombine for the case, it would provide mul nuw i32 %iv, 900000000:
https://godbolt.org/z/6oWrPbEMn

[topperc]

I guess we need to clear the flags from every mul we changed, and any add we passed through to reach that mul. The FP flags are probably also wrong but harder to observe since most FP nodes have the same set of flags in normal usage.

CC @nikic @preames