inliner generates invalid OpLabel if callee has single return in block that does not appear last

[dneto0]

Example code:

               OpCapability Shader
          %1 = OpExtInstImport "GLSL.std.450"
               OpMemoryModel Logical GLSL450
               OpEntryPoint Vertex %main "main"
               OpSource GLSL 450
               OpName %main "main"
               OpName %foo_ "foo("
       %void = OpTypeVoid
          %3 = OpTypeFunction %void
       %bool = OpTypeBool
       %true = OpConstantTrue %bool
       %main = OpFunction %void None %3
          %5 = OpLabel
         %12 = OpFunctionCall %void %foo_
               OpReturn
               OpFunctionEnd
       %foo_ = OpFunction %void None %3
          %7 = OpLabel
               OpSelectionMerge %11 None
               OpBranchConditional %true %10 %11
         %11 = OpLabel
               OpReturn  ;  This causes returnLabelId to be non-zero, but earlyReturn is false.
         %10 = OpLabel
               OpBranch %11
               OpFunctionEnd

It's generated from this vertex shader, but where I reorder, in the SPIR-V assembly, the last two basic blocks of the "foo" function:

#version 450

void foo() {
 if (true) {
 }
}
void main() {
  foo();
}

When running spirv-opt with inline-entry-points-exhaustive:

• returnLabelId is non-zero since we encountered an OpReturn well before the OpFunctionEnd
• but earlyReturn is false
• but at OpFunctionEnd we generate "continue target" with OpLabel for result id singleTripLoopContinueId. But that is still zero.
• this causes a new instruction with OpLabel opcode, but zero result Id, and generates a 1-word OpLabel instruction, which is invalid.

Likely remedy is to guard the codegen of continue target block by earlyReturn. Still have to create the block with label Id returnLabelId.

[dneto0]

I expect to have patch soon.
Cc @greg-lunarg

[dneto0]

I have a code fix at https://github.com/dneto0/SPIRV-Tools/tree/inline-avoid-nil-label
Need to make a test for it.