Add a C test for peeling.

Previously it was impractical for us to write a test for peeling:
limitations in the optimiser meant that you needed a complex set of
things to go right for peeling to end up having visibly good effects.
The previous commit to this has made it possible for us to write a small
test that makes the effect of peeling obvious: in this case we are able
to elide a guard in the body.

tests/c/peel1.c

@@ -0,0 +1,55 @@
+// ## The sanitisers fiddle with the generated code and mean we can't write the
+// ## test we want.
+// ignore-if: echo $RUSTFLAGS | grep "sanitizer"
+// Compiler:
+//   env-var: YKB_EXTRA_CC_FLAGS=-O2
+// Run-time:
+//   env-var: YKD_SERIALISE_COMPILATION=1
+//   env-var: YKD_LOG_IR=jit-post-opt
+//   stderr:
+//     0 1
+//     --- Begin jit-post-opt ---
+//     ...
+//     guard true, ...
+//     ...
+//     guard true, ...
+//     header_end [1i32, ...
+//     ...
+//     body_start ...
+//     ...
+//     guard true, ...
+//     body_end [1i32, ...
+//     --- End jit-post-opt ---
+//     1 1
+//     2 1
+//     3 1
+//     4 1
+
+// Check that peeling works: a constant should be discovered by `header_end`
+// that allows the body to have only 1 guard where the header must have 2.
+
+#include <assert.h>
+#include <inttypes.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <yk.h>
+#include <yk_testing.h>
+
+int main(int argc, char **argv) {
+  YkMT *mt = yk_mt_new(NULL);
+  yk_mt_hot_threshold_set(mt, 0);
+  YkLocation loc = yk_location_new();
+
+  int i = 0;
+  NOOPT_VAL(i);
+  for (; i < 5; i++) {
+    yk_mt_control_point(mt, &loc);
+    int y = yk_promote(argc);
+
+    fprintf(stderr, "%d %d\n", i, y);
+  }
+
+  yk_location_drop(loc);
+  yk_mt_shutdown(mt);
+  return (EXIT_SUCCESS);
+}