[llvm] [M68k] add test showing callseq begin failure for doubles, and fix mem chain generation (PR #170049)

[Min-Yih Hsu via llvm-commits]

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@@ -584,7 +585,8 @@ void ScheduleDAGRRList::ReleasePredecessors(SUnit *SU) {
         unsigned NestLevel = 0;
         unsigned MaxNest = 0;
         SDNode *N = FindCallSeqStart(Node, NestLevel, MaxNest, TII);
-        assert(N && "Must find call sequence start");
+        if (!N)
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mshockwave wrote:

if we bail out upon failing to find callseq_start, then we might not be able to prevent other instructions from getting between callseq_start & call instruction & callseq_end

https://github.com/llvm/llvm-project/pull/170049