Collect SwitchInt target VariantIdxs while building MaybePlacesSwitchIntData

[ashivaram23]

• Filter discriminants involved in a SwitchInt and map their values to VariantIdx while building MaybePlacesSwitchIntData instead of in apply_effects_in_block using next_discr. This is easier after Remove support for SwitchInt edge effects in backward dataflow #143769.
• Use that Vec when handling the otherwise target instead of making a new SmallVec.

This could make MaybeInitializedPlaces/MaybeUninitializedPlaces analysis slightly more efficient, particularly after #142707.

[rustbot]

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[ashivaram23]

Note: I don't imagine this will have a major perf impact unless there's an enum with a hundred variants. I also don't expect this to fix the regressions from #142707 which I believe are largely from how the generated code interacts with later passes and LLVM.

[compiler-errors]

Let's see what perf says anyways

@bors2 try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[rust-bors]

⌛ Trying commit 04c8592 with merge 8494e68…

To cancel the try build, run the command @bors2 try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 8494e68 (8494e6881d8ac4d5f15e3ed5b25fc7c1cac5a7f8, parent: bfc046a4b8d6b57db02540182466e989a9b0fb40)

[rust-timer]

Queued 8494e68 with parent bfc046a, future comparison URL.
There are currently 3 preceding artifacts in the queue.
It will probably take at least ~3.9 hours until the benchmark run finishes.