Rollup merge of #143410 - scottmcm:redo-transmute-again, r=RalfJung,workingjubilee

Block SIMD in transmute_immediate; delete `OperandValueKind`

Vectors have been causing me problems for years in this code, for example https://github.com/rust-lang/rust/pull/110021#discussion_r1160975086 and https://github.com/rust-lang/rust/pull/143194

See conversation in <https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Is.20transmuting.20a.20.60T.60.20to.20.60Tx1.60.20.28one-element.20SIMD.20vector.29.20UB.3F/near/526262799>.

By blocking SIMD in `transmute_immediate` it can be simplified to just take the `Scalar`s involved -- the backend types can be gotten from those `Scalar`s, rather than needing to be passed.  And there's an assert added to ICE it if it does get hit.

Accordingly, this changes `rvalue_creates_operand` to not send SIMD transmutes through the operand path, but to always go through memory instead, like they did back before rust-lang/rust#108442.

And thanks to those changes, I could also remove the `OperandValueKind` type that I added back then which `@RalfJung` rightly considers pretty sketchy.

cc `@folkertdev` `@workingjubilee` from the zulip conversation too

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