Replace InsertLane format with TernaryImm8

[abrown]

The InsertLane format has an ordering (value().imm().value()) and immediate name ("lane") that make it awkward to use for other instructions. This changes the ordering (value().value().imm()) and uses the default name ("imm") throughout the codebase.

[abrown]

I am also thinking of doing the same for ExtractLane--it's a bit too specific--but that can be a separate PR.

[github-actions]

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[iximeow]

I feel it's important to note here that this is a pretty x86-oriented change - do NEON instructions have similar "select-a-lane-of-the-dest-with-an-immediate" instructions? It seems like the alternative is to have different formats for different place imm8 makes sense in clif IR, which is admittedly not great..

edit: NEON instructions would go through the new backend machinery anyway, so that question isn't so relevant.

I don't want to single-handedly encourage orienting the old backend machinery towards x86, so: this seems like a fair change in considering other reg, reg, imm-encoded SIMD instructions but I'd like a second ✔️ on the broader idea of this code really just supporting x86 encoding at this point.

[abrown]

Closing this one in favor of #1770 after chatting offline with @iximeow.