Add LLVM crash case.

suite/regress/c-crashers/crash-28-x64-llvm-error-unable-to-evaluate-offset-for-variable.c

+#include <keystone/keystone.h>
+int main(int argc, char **argv) {
+  int ks_arch = KS_ARCH_X86, ks_mode = KS_MODE_64;
+  unsigned char assembly[] = {
+    'x', '=', 'y', '-', '-', 'y', ';', '.', '=', 'x',
+    '-', 'y', 0x00,
+  };
+  ks_engine *ks;
+  ks_err err = ks_open(ks_arch, ks_mode, &ks);
+  if (!err) {
+    size_t count, size;
+    unsigned char *insn;
+    if (ks_asm(ks, (char *)assembly, 0, &insn, &size, &count))
+      printf("ERROR: failed on ks_asm() with error = %s, code = %u\n", ks_strerror(ks_errno(ks)), ks_errno(ks));
+    ks_free(insn);
+  }
+  ks_close(ks);
+  return 0;
+}