SplitBB machine (#1811)

Very copy-pastey stuff.

test_data/std/split_bb_test.asm

@@ -0,0 +1,49 @@
+use std::machines::split::ByteCompare;
+use std::machines::split::split_bb::SplitBB;
+
+machine Main with degree: 65536 {
+    reg pc[@pc];
+    reg X0[<=];
+    reg X1[<=];
+    reg X2[<=];
+    reg low;
+    reg high;
+
+    ByteCompare byte_compare;
+    SplitBB split_machine(byte_compare);
+
+    instr split X0 -> X1, X2 link ~> (X1, X2) = split_machine.split(X0);
+
+    instr assert_eq X0, X1 {
+        X0 = X1
+    }
+
+    function main {
+
+        // Min value
+        // Note that this has two byte decompositions, 0 and p = 0x78000001.
+        // The second would lead to a different split value, but should be ruled
+        // out by the overflow check.
+        low, high <== split(0);
+        assert_eq low, 0;
+        assert_eq high, 0;
+
+        // Max value
+        // On BabyBear, this is 0x78000000.
+        low, high <== split(-1);
+        assert_eq low, 0;
+        assert_eq high, 0x7800;
+
+        // Max low value
+        low, high <== split(0x77ffffff);
+        assert_eq low, 0xffff;
+        assert_eq high, 0x77ff;
+
+        // Some other value
+        low, high <== split(0x42abcdef);
+        assert_eq low, 0xcdef;
+        assert_eq high, 0x42ab;
+
+        return;
+    }
+}