spv/lift: elide empty targets of an if-else/switch, as allowed by SPIR-V.

[eddyb]

SPIR-V allows what it calls "merge edges" (edges from a structured header block, to its merge block), e.g.:

if cond {
    asm!("OpKill");
}
// ...

can directly use the merge block as the "else" edge of the conditional branch:

  OpSelectionMerge %merge None
  OpBranchConditional %cond %then %merge

%then = OpLabel
  OpKill

%merge = OpLabel
  ; ...

but before this PR, SPIR-T -> SPIR-V lifting was not taking advantage of that, instead emitting:

  OpSelectionMerge %merge None
  OpBranchConditional %cond %then %else

%then = OpLabel
  OpKill

%else = OpLabel
  OpBranch %merge

%merge = OpLabel
  ; ...

---

This pattern became most obvious on Rust-GPU shaders with a lot of panics from e.g. bounds-checking (which get turned into conditional "OpReturn from entry point"), but not from inspecting the SPIR-V output.

Instead, I ended up noticing that spirv-opt has a merge-blocks pass, which can remove the slight inefficiency from situations like the above example, but it's very slow for what it does (and it was the slowest pass in spirv-opt --time-report, for the SPIR-V module I was looking at).
_{(if linear, that would mean around 200µs per trivial basic block, which feels absurd given how simple the check in this PR is, so I suspect it's accidentally quadratic or worse, but I haven't looked at the C++ code to confirm either)}

EDIT: with this PR used in Rust-GPU, the specific testcase which prompted it (schell/renderling@d9f4d6f) has the time spent in spirv-opt shrink by ~20% (4s -> 3.21s), which is even better than the 0.5s I was expecting.

[LegNeato]

Do we have tests? This seems super testable.

[LegNeato]

Let's put an error or panic in here so if we hit it we are aware.