Auto merge of #126784 - scottmcm:smaller-terminator, r=compiler-errors

Save 2 pointers in `TerminatorKind` (96 → 80 bytes)

These things don't need to be `Vec`s; boxed slices are enough.

The frequent one here is call arguments, but MIR building knows the number of arguments from the THIR, so the collect is always getting the allocation right in the first place, and thus this shouldn't ever add the shrink-in-place overhead.

Author: bors