Avoid some allocas in codegen of CastKind::Transmute

[scottmcm]

Now that it's a Cast instead of a Call, thanks to #108442, we don't need to write out the value to a place every time.

For simple cases like transmuting to/from a newtype, we can instead just pass along the immediate SSA value.

[rustbot]

r? @lcnr

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[lcnr]

r? compiler

[b-naber]

r? @oli-obk

[scottmcm]

I'm going to close this for now, and probably come back to it with a different approach so long as #109556 goes well.