Repro case reduced much further:

; ModuleID = ''
source_filename = "bugpoint-output-a3b3e72.bc"
target datalayout = "e-m:e-i64:64-n32:64"
target triple = "powerpc64le-unknown-linux-gnu"

; Function Attrs: norecurse nounwind
define void @​bn_mul_comba8(i64* nocapture %r, i64* nocapture readonly %a, i64* nocapture readonly %b) local_unnamed_addr #​0 {
%1 = load i64, i64* %a, align 8
%conv = zext i64 %1 to i128
%2 = load i64, i64* %b, align 8
%conv2 = zext i64 %2 to i128
%mul = mul nuw i128 %conv2, %conv
%shr = lshr i128 %mul, 64
%agep = getelementptr inbounds i64, i64* %a, i64 1
%3 = load i64, i64* %agep, align 8
%conv14 = zext i64 %3 to i128
%mul15 = mul nuw i128 %conv14, %conv
%add17 = add i128 %mul15, %shr
%shr19 = lshr i128 %add17, 64
%conv20 = trunc i128 %shr19 to i64
%bgep = getelementptr inbounds i64, i64* %b, i64 1
%4 = load i64, i64* %bgep, align 8
%conv28 = zext i64 %4 to i128
%mul31 = mul nuw i128 %conv28, %conv2
%conv32 = and i128 %add17, 18446744073709551615
%add33 = add i128 %conv32, %mul31
%shr35 = lshr i128 %add33, 64
%conv36 = trunc i128 %shr35 to i64
%add37 = add i64 %conv36, %conv20
%cmp38 = icmp ult i64 %add37, %conv36
%conv148 = zext i1 %cmp38 to i64
store i64 %conv148, i64* %r, align 8
ret void
}

attributes #​0 = { norecurse nounwind }

Faulty DAG
And the faulty DAG. It seems like the problem is that some glue is not schedulable. Any idea how to fix this ? What are other backend doing ?