Generate more fixed_nonallocatable constraints, and add debug assertions

[elliottt]

Add assertions to the OperandCollector that show we're not using pinned vregs, and use reg_fixed_nonallocatable constraints when a real register is used with other constraint generation functions like reg_use etc.

I experimented with adding debug assertions that the reused operand referenced in reg_reuse_def constraints was compatible with the new constraint that would be generated. This didn't end up being possible as some backends will recursively process instructions when collecting operands, making it impossible without more invasive changes to tell where that operand is in the operand vector. Given that we would need to plumb through somewhat substantial changes to support adding debug assertions, I decided that rephrasing the existing warning comment about real registers would be good enough.

[github-actions[bot]]

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