GH-121784: Generate an error during code gen if a variable is marked unused, but is used and thus cached in a prior uop.

[markshannon]

• Issue: Tier 1 code generator doesn't track used values if named unused #121784

[Fidget-Spinner]

I'll review after tests pass.

[markshannon]

This ended up being rather involved. The explicit stores to the stack were needed not just because the code generator failed to track values across uops with unused operands, but because the stack needs to be flushed to memory, in case of a de-opt.
I've fixed that by adding support for an explicit flush operation in macros.

[Fidget-Spinner]

Just one comment, else LGTM. I've taken a look at the generated code for tier 1 as well.