[FRONTEND] Semantic analysis refactor (#491)

Moved dispatch.cc to semantic.py (@ptillet)
Integer signedness analysis was moved from C++ to python (@daadaada)
Cleaner frontend types (@daadaada)
Moved SSA construction to a separate object (@ptillet)


Co-authored-by: Yan Da <dyanab@connect.ust.hk>

python/test/regression/test_performance.py

@@ -37,7 +37,7 @@ matmul_data = {
         (256, 256, 256): {'float16': 0.027},
         (512, 512, 512): {'float16': 0.158},
         (1024, 1024, 1024): {'float16': 0.466},
-        (2048, 2048, 2048): {'float16': 0.680},
+        (2048, 2048, 2048): {'float16': 0.695},
         (4096, 4096, 4096): {'float16': 0.831},
         (8192, 8192, 8192): {'float16': 0.849},
         # tall-skinny