Reduces LUT needed by mul extension

bench/tests/op/mulhu.s

@@ -1,20 +1,54 @@
-# expected: 0000000000000000, 0000000000000000, c1b080e9, deacf, deacf
     .text
 _start:
-    li x2, 0xdead0
-    li x3, 0x0
-    li x4, 0x1
-    li x5, (-1)
-    li x6, 0xdead0
-    li x7, 0xdead00000000
+    li x2,  0xdead00000        # +59774074880
+    li x3,  0              # zero
+    li x4,  1              # one
+    li x5,  123456789          # small positive
+    li x6,  0xdead00000        # duplicate of x2
+    li x7,  765432000000        # large positive
+    li x8,  123456789          # small positive
+    li x9, 234567890000        # large positive
+    li x10, 0xffffffffffffffff  # max positive 64b
+    li x11, 0x8000000000000000  # min unsigned = 2^63
 
-    # Mul 0
+    # --------------------------------------------------------------
+    # 1. MULHU : a * 0
     mulhu x31, x2, x3
+    # expected: 0000000000000000
 
-    # Basic multiplications
+    # 2. MULHU : a * 1
     mulhu x31, x2, x4
-    mulhu x31, x7, x7
+    # expected: 0000000000000000
 
-    # Overflow
-    mulhu x31, x5, x2
-    mulhu x31, x2, x5
+    # 3. MULHU : a * a
+    mulhu x31, x2, x6
+    # expected: 00000000000000c1
+
+    # 4. MULHU : small * small
+    mulhu x31, x5, x8
+    # expected: 0000000000000000
+
+    # 5. MULHU : small * large
+    mulhu x31, x5, x7
+    # expected: 0000000000000005
+
+    # 6. MULHU : large * small
+    mulhu x31, x7, x8
+    # expected: 0000000000000005
+
+    # 7. MULHU : large * large
+    mulhu x31, x7, x9
+    # expected: 0000000000002605
+
+    # 8. MULHU : max * 2
+    li x12, 2
+    mulhu x31, x10, x12
+    # expected: 0000000000000001
+
+    # 9. MULHU : max * max
+    mulhu x31, x10, x10
+    # expected: fffffffffffffffe
+
+    # 10. MULHU : min * max
+    mulhu x31, x11, x10
+    # expected: 7fffffffffffffff