[llvm] [M68k] CallSeq assertion failure in SelectionDAG

[knickish]

; RUN: llc < %s -mtriple=m68k-linux | FileCheck %s

define double @minimized() {
start:
  %_58 = call double null(double 0.000000e+00)
  %_60 = load double, ptr null, align 8
  %_57 = fmul double 0.000000e+00, %_60
  store double %_57, ptr null, align 8
  ret double 0.000000e+00
}

fails with .../bin/FileCheck .../llvm/lib/CodeGen/SelectionDAG/ScheduleDAGRRList.cpp:510: llvm::SDNode* FindCallSeqStart(llvm::SDNode*, unsigned int&, unsigned int&, const llvm::TargetInstrInfo*): Assertion 'Best' failed.

Something appears to be wrong with the callseq_start/end ordering with these instructions, but not yet sure what the exact problem is

[llvmbot]

@llvm/issue-subscribers-backend-m68k

Author: None (knickish)

```ll ; RUN: llc < %s -mtriple=m68k-linux | FileCheck %s

define double @minimized() {
start:
%_58 = call double null(double 0.000000e+00)
%_60 = load double, ptr null, align 8
%_57 = fmul double 0.000000e+00, %_60
store double %_57, ptr null, align 8
ret double 0.000000e+00
}

fails with `.../bin/FileCheck .../llvm/lib/CodeGen/SelectionDAG/ScheduleDAGRRList.cpp:510: llvm::SDNode* FindCallSeqStart(llvm::SDNode*, unsigned int&, unsigned int&, const llvm::TargetInstrInfo*): Assertion 'Best' failed.`

Something appears to be wrong with the `callseq_start/end` ordering with these instructions, but not yet sure what the exact problem is


</details>

[llvmbot]

@llvm/issue-subscribers-bug

Author: None (knickish)

```ll ; RUN: llc < %s -mtriple=m68k-linux | FileCheck %s

define double @minimized() {
start:
%_58 = call double null(double 0.000000e+00)
%_60 = load double, ptr null, align 8
%_57 = fmul double 0.000000e+00, %_60
store double %_57, ptr null, align 8
ret double 0.000000e+00
}

fails with `.../bin/FileCheck .../llvm/lib/CodeGen/SelectionDAG/ScheduleDAGRRList.cpp:510: llvm::SDNode* FindCallSeqStart(llvm::SDNode*, unsigned int&, unsigned int&, const llvm::TargetInstrInfo*): Assertion 'Best' failed.`

Something appears to be wrong with the `callseq_start/end` ordering with these instructions, but not yet sure what the exact problem is


</details>