llvm dev - Jun 2017 - reg alloc bug? "Different value live out of predecessor"

I'm experiencing a few related assert-failures in the register allocator. 
Unfortunately, I can only reproduce these in my out-of-tree backend :(  I'm
looking for advice how to debug these failures.

-verify-regalloc exposes the error.   It occurs when RAGreedy::splitAroundRegion
calls MF->verify("After splitting live range around region").  The
error report says "Different value live out of predecessor."  I
believe that the error pertains to this to %vreg190 defined below---a 
particularly ugly IMPLICIT_DEF of a lower lane of a 2xi32 vector register.

Any tips would be appreciated
Nick Johnson
D. E. Shaw Research


...
4144B   BB#47: derived from LLVM BB %bb158
            Predecessors according to CFG: BB#46
4160B           %vreg19<def> = INSTR_LA_S__I <ga:@baz.607>;
ScalarRegs:%vreg19
4176B           %vreg190:sub_32_1<def,read-undef> = IMPLICIT_DEF;
PairRegs:%vreg190
            Successors according to CFG: BB#48(?%)

4208B   BB#48: derived from LLVM BB %bb162
            Predecessors according to CFG: BB#46 BB#47
4256B           INSTR_SW__S_I_S %vreg166, %R0_0, 782064; mem:ST4[undef](tbaa=!8)
ScalarRegs:%vreg166
4328B           %vreg176<def> = INSTR_LW_S__I_S <fi#2>, 0;
mem:LD4[FixedStack2] ScalarRegs:%vreg176
4332B           %vreg191<def> = COPY %vreg190; PairRegs:%vreg191,%vreg190
4336B           INSTR_BNE__S_I_I_I__S_U__TARGET_MUL_4 %R0_0, 0, <BB#11>, 0
4352B           INSTR_J__I_I__U__TARGET_MUL_4 <BB#53>, 0
            Successors according to CFG: BB#53(0x7ffff800 / 0x80000000 =
100.00%) BB#11(0x00000800 / 0x80000000 = 0.00%)
...
*** Bad machine code: Different value live out of predecessor ***
- function:    ham.928
- basic block: BB#47 bb158 (0x471ee70) [4144B;4208B)
- liverange:   [2280r,3424B:0)[3440B,4144B:0)[4208B,4332r:0)  0 at 2280r
- v. register: %vreg190
- lanemask:    00000001

At which point MachineVerifier::verifyLiveRangeSegment() segfaults as it
appempts to dereference PVNI->id with null PVNI.

Valno #4294967295 live out of BB#47 at 4208B
Valno #0 live into BB#48 at 4208B
LLVM ERROR: Found 1 machine code errors.