spirv-opt: MergeReturn pass places DebugFunctionDefinition in non-first block of a function.

[csyonghe]

Given this SPIRV Input:

in.zip

Running

spirv-val in.spv --scalar-block-layout

returns no errors.

However, if we run the merge return pass on this spirv and then validate its output, we will get an error:

spirv-opt in.spv --scalar-block-layout --merge-return -o out.spv
spirv-val out.spv --scalar-block-layout

Produces:

error: line 171: NonSemantic.Shader.DebugInfo.100 DebugFunctionDefinition: must be in the entry basic block of the function
  %38 = OpExtInst %void %2 DebugFunctionDefinition %34 %main

Observed with SPIRV-Tools v2024.4 v2024.4.rc2-0-g4d2f0b40.