Rollup merge of #142044 - workingjubilee:document-operandvalue-pair, r=scottmcm

compiler: Document the offset invariant of `OperandValue::Pair`

A subtle invariant of `OperandValue::Pair` that came up during review and was found to be undocumented.

Visible in code like this:
https://github.com/rust-lang/rust/blob/4b27a04cc8ed4da10a546a871e23e665d03f7a79/compiler/rustc_codegen_ssa/src/mir/operand.rs#L376-L392

Author: matthiaskrgr