`OpLoopMerge` emission was not supported due to the fact that spirv
structurizer is not being run for non-shader targets.
After enabling support for `SPV_INTEL_unstructured_loop_controls` in
https://github.com/llvm/llvm-project/pull/178799 is started to preserve
some information about unstructured control flow. This PR is intended to
enable support for `OpLoopMerge` without extension.
Note: changes in `llvm/test/CodeGen/SPIRV/pointers/phi-chain-types.ll`
and `llvm/test/CodeGen/SPIRV/llvm-intrinsics/memset.ll` are due to the
fact that loop layout has changed after `loop-simplify` pass enabling
4a773b9f35