Opt: segfault or assertion failure on merge blocks for unreachable loop #2374
Description
The following SPIRV causes an assertion failure in spirv-opt. The issue is that the merge blocks pass tries to merge blocks %15 and %11 via the back-edge. The loop is unreachable. I came across this issue using spirv-reduce, which uses the merge blocks code. I made a test using the text below, plus: SinglePassRunAndCheck<BlockMergePass>(text, text, true, true); In other words, I assume there should be no change once the issue is fixed.
spirv-opt --merge-blocks -o opt.spv in.spv
OpCapability Shader
%1 = OpExtInstImport "GLSL.std.450"
OpMemoryModel Logical GLSL450
OpEntryPoint Fragment %main "main"
OpExecutionMode %main OriginUpperLeft
OpSource ESSL 310
OpName %main "main"
%void = OpTypeVoid
%4 = OpTypeFunction %void
%int = OpTypeInt 32 1
%_ptr_Function_int = OpTypePointer Function %int
%bool = OpTypeBool
%false = OpConstantFalse %bool
%main = OpFunction %void None %4
%9 = OpLabel
OpBranch %10
%11 = OpLabel
OpLoopMerge %12 %13 None
OpBranchConditional %false %13 %14
%13 = OpLabel
OpSelectionMerge %15 None
OpBranchConditional %false %16 %17
%16 = OpLabel
OpBranch %15
%17 = OpLabel
OpBranch %15
%15 = OpLabel
OpBranch %11
%14 = OpLabel
OpReturn
%12 = OpLabel
OpBranch %10
%10 = OpLabel
OpReturn
OpFunctionEnd