optimization for if/break as first statement in a loop body

for(...; x; ...) if (y) break; → for(...; x&&!y; ...);

similarly for `while` and some combinations (i.e. the `break` appears in the
`else` clause, etc.)

test/compress/loops.js

@@ -0,0 +1,123 @@
+while_becomes_for: {
+    options = { loops: true };
+    input: {
+        while (foo()) bar();
+    }
+    expect: {
+        for (; foo(); ) bar();
+    }
+}
+
+drop_if_break_1: {
+    options = { loops: true };
+    input: {
+        for (;;)
+            if (foo()) break;
+    }
+    expect: {
+        for (; !foo(););
+    }
+}
+
+drop_if_break_2: {
+    options = { loops: true };
+    input: {
+        for (;bar();)
+            if (foo()) break;
+    }
+    expect: {
+        for (; bar() && !foo(););
+    }
+}
+
+drop_if_break_3: {
+    options = { loops: true };
+    input: {
+        for (;bar();) {
+            if (foo()) break;
+            stuff1();
+            stuff2();
+        }
+    }
+    expect: {
+        for (; bar() && !foo();) {
+            stuff1();
+            stuff2();
+        }
+    }
+}
+
+drop_if_break_4: {
+    options = { loops: true, sequences: true };
+    input: {
+        for (;bar();) {
+            x();
+            y();
+            if (foo()) break;
+            z();
+            k();
+        }
+    }
+    expect: {
+        for (; bar() && (x(), y(), !foo());) z(), k();
+    }
+}
+
+drop_if_else_break_1: {
+    options = { loops: true };
+    input: {
+        for (;;) if (foo()) bar(); else break;
+    }
+    expect: {
+        for (; foo(); ) bar();
+    }
+}
+
+drop_if_else_break_2: {
+    options = { loops: true };
+    input: {
+        for (;bar();) {
+            if (foo()) baz();
+            else break;
+        }
+    }
+    expect: {
+        for (; bar() && foo();) baz();
+    }
+}
+
+drop_if_else_break_3: {
+    options = { loops: true };
+    input: {
+        for (;bar();) {
+            if (foo()) baz();
+            else break;
+            stuff1();
+            stuff2();
+        }
+    }
+    expect: {
+        for (; bar() && foo();) {
+            baz();
+            stuff1();
+            stuff2();
+        }
+    }
+}
+
+drop_if_else_break_4: {
+    options = { loops: true, sequences: true };
+    input: {
+        for (;bar();) {
+            x();
+            y();
+            if (foo()) baz();
+            else break;
+            z();
+            k();
+        }
+    }
+    expect: {
+        for (; bar() && (x(), y(), foo());) baz(), z(), k();
+    }
+}