Add Om1 lowering with no optimizations

crosstest/simple_loop_main.c

+/* crosstest.py --test=simple_loop.c --driver=simple_loop_main.c \
+   --prefix=Subzero_ --output=simple_loop */
+
+#include <stdio.h>
+
+int simple_loop(int *a, int n);
+int Subzero_simple_loop(int *a, int n);
+
+int main(int argc, char **argv) {
+  unsigned TotalTests = 0;
+  unsigned Passes = 0;
+  unsigned Failures = 0;
+  int a[100];
+  for (int i = 0; i < 100; ++i)
+    a[i] = i * 2 - 100;
+  for (int i = -2; i < 100; ++i) {
+    ++TotalTests;
+    int llc_result = simple_loop(a, i);
+    int sz_result = Subzero_simple_loop(a, i);
+    if (llc_result == sz_result) {
+      ++Passes;
+    } else {
+      ++Failures;
+      printf("Failure: i=%d, llc=%d, sz=%d\n", i, llc_result, sz_result);
+    }
+  }
+  printf("TotalTests=%u Passes=%u Failures=%u\n", TotalTests, Passes, Failures);
+  return Failures;
+}