[rustc_codegen_ssa] Let function calls and definitions use OperandValue or custom enum instead of Bx::Value

[bjorn3]

For example Bx::Value is assumed to be able to contain a pair

I know this is offtopic but I just wanted to note that pair creation is intentionally funnelled through methods with "immediate_or_packed_pair" in their name, and they're limited to function inputs/outputs, for both calls and definitions (including e.g returns).
The solution there is to have an enum (or reuse OperandValue) which is used for e.g. call, invoke, ret and function params, but nothing else.

Originally posted by @eddyb in #65881 (comment)

This is necessary to use cg_ssa with backends which can't represent pairs in Bx::Value.

[bjorn3]

@rustbot modify labels:+A-cranelift

This is one step closer towards using cg_ssa in cg_clif.