NewGVN optimizes:

define i1 @test({}** %arg, i1 %arg2) {
    br i1 %arg2, label %bb1, label %bb2

bb1:
    %load1 = load {}*, {}** %arg
    %cmp1 = icmp eq {}* %load1, null
    ret i1 %cmp1

bb2:
    %load2 = load {}*, {}** %arg, !nonnull !1
    %cmp2 = icmp eq {}* %load2, null
    ret i1 %cmp2
}

!1 = !{}

into:

define i1 @test({}** %arg, i1 %arg2) {
  br i1 %arg2, label %bb1, label %bb2

bb1:                                              ; preds = %0
  ret i1 false

bb2:                                              ; preds = %0
  ret i1 false
}

The reason is that %load1 and %load2 are found congruent with %load2 chosen as leader. Thus %cmp1 in bb1 calls SimplifyCmp with icmp eq {}* %load2, null, which is simplified to i1 false based on the !nonnull metadata of %load2. However, this metadata is not applicable in this branch.

This bug prevents bootstrapping of rustc under newgvn.