Float: Introduce nan propagation for same exp add

[Incarnation-p-lee]

• Add nan propagation when ops has same exp.
• Refine the high level layout of add.
• Add assertion for sub function.
• Make sure the nan propagation covered by test.

[github-actions[bot]]

Test Results

   10 files  ±0     22 suites  ±0   0s ⏱️ ±0s   716 tests ±0    716 ✅ ±0  0 💤 ±0  0 ❌ ±0  2 247 runs  ±0  2 246 ✅ ±0  1 💤 ±0  0 ❌ ±0 

Results for commit 3e6a44ea. ± Comparison against base commit 87dddf82.