spirv-opt: DominatorAnalysis is not invalidated but the CFG is

[Vasniktel]

MergeReturnPass invalidates CFG but doesn't invalidate dominator trees here

SPIRV-Tools/source/opt/merge_return_pass.cpp

Lines 429 to 431 in b78f4b1

	// Make sure the CFG is build here. If we don't then it becomes very hard
	// to know which new blocks need to be updated.
	context()->BuildInvalidAnalyses(IRContext::kAnalysisCFG);

Dominator trees depend on CFG's pseudo entry and exit blocks. When CFG is invalidated, dominator trees contain dangling pointers. This causes assertion failures later in the program.

The bug was found by inspecting an assertion failure in the attached shader. The failure happens in

SPIRV-Tools/source/opt/basic_block.h

Lines 119 to 120 in b78f4b1

	iterator tail() {
	assert(!insts_.empty());

when one tries to run spirv-opt <attached file> -o out.spv --validate-after-all --if-conversion --merge-return. --if-conversion is used only to initialize dominator trees before --merge-return is executed.

data.zip contains a GLSL shader and its compiled SPIR-V version.

Can be reproduced on b78f4b1.

[Vasniktel]

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