RegisterCoalescer: Do not introduce uses of empty register classes (#161809) Check RegisterClassInfo if any registers of the new class are actually available for use. Currently AMDGPU overrides shouldCoalesce to avoid this situation. The target hook does not have access to the dynamic register class counts, but ideally the target hook would only be used for profitability concerns. The new test doesn't change, due to the AMDGPU shouldCoalesce override, but would be unallocatable if we dropped the override and switched to the default implementation. The existing limit-coalesce.mir already tests the behavior of this override, but it's too conservative and isn't checking the case where the new class is unallocatable. Add this check so it can be relaxed.
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