Fix/implement checked_{add,sub,mul} and saturating_{add,sub} (using SPIR-V instructions were possible).

[eddyb]

After this PR:

• signed checked_{add,sub} still have no direct SPIR-V equivalents
  • our custom implementation used to have bugs around e.g. 0 + 0 / 0 - 0
    (fixed in this PR, and verified via Alive2: broken vs correct)
• unsigned checked_{add,sub} and signed/unsigned checked_mul map to SPIR-V instructions
  (OpIAddCarry, OpISubBorrow, OpSMulExtended/OpUMulExtended)
• saturating_{add,sub} use an Intel extension instead of plainly erroring
  (OpIAddSatINTEL/OpISubSatINTEL)
  • in time, we might choose to emulate these instructions (later in the pipeline)

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TODO: add tests for the edge cases, consider the need for OpUAddSatINTEL/OpUSubSatINTEL.

[nazar-pc]

Could you also comment on #403? Seems directly applicable here.