(In reply to Grigori Goronzy from comment #9)
> Created attachment 115409 [details] [review] [review]
> Full shader
> 
> That does not help either. Seems to break the SSA somehow. Full shader that
> triggers the bug attached.
> 
> The patch I posted earlier help with the the reduced testcase, but not with
> the full shader. Both undef incoming values in phi nodes and undef branch
> conditions cause problems in different ways.

With the second patch + valley I get -

valley_x64: LiveVariables.cpp:114: void
llvm::LiveVariables::MarkVirtRegAliveInBlock(llvm::LiveVariables::VarInfo &,
llvm::MachineBasicBlock *, llvm::MachineBasicBlock *,
std::vector<MachineBasicBlock *> &): Assertion `MBB != &MF->front() && "Can't
find reaching def for virtreg"' failed.