Revert "Use the HIR instead of mir_keys for determining whether something will have a MIR body."
This reverts commit e5cba17b84bf7bf755686e8bb36aa3775ef53f77.
turns out SMIR still needs it (https://github.com/model-checking/kani/issues/3218). I'll create a full plan and MCP for what I intended this to be a part of. Maybe my plan is nonsense anyway.