a BIT of 65C02 optimisations

Use BIT immediate instead of AND when reloading A is required
afterwards.
Add an fread unit test as the optimisation touches fread.
Sprinkle a few zero page indexed while we're at it.

test/ref/test_fread.c

@@ -0,0 +1,70 @@
+/*
+  !!DESCRIPTION!! fgets test
+  !!LICENCE!!     Public domain
+*/
+
+#include "common.h"
+
+#include <ctype.h>
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+FILE *in, *out;
+char buf[32];
+
+#define INFILE   "cf.in"
+
+int main(int argc,char **argv)
+{
+    static char outfile_path[FILENAME_MAX+1];
+    static int r;
+
+    sprintf(outfile_path, "%s.test.out", argv[0]);
+
+    out = fopen(outfile_path, "wb");
+    if (out == NULL) {
+        return EXIT_FAILURE;
+    }
+    if (fread(buf, 1, sizeof(buf), out) != NULL) {
+      printf("Error, could fread with write-only file\n");
+      return 1;
+    }
+    if (!ferror(out)) {
+      printf("Error: file pointer should be in error state\n");
+    }
+    fclose(out);
+    unlink(outfile_path);
+
+    in = fopen(INFILE, "rb");
+    if (in == NULL) {
+        return EXIT_FAILURE;
+    }
+
+    /* Test that ungetc doesn't break fread */
+    buf[0] = fgetc(in);
+    ungetc(buf[0], in);
+
+    r = fread(buf, 1, sizeof(buf), out);
+
+    if (r == 0) {
+      printf("Error: could not start reading.\n");
+    }
+    fwrite(buf, 1, r, stdout);
+
+    /* Finish reading file. */
+    while ((r = fread(buf, 1, sizeof(buf), out)) != 0)
+    {
+        fwrite(buf, 1, r, stdout);
+    }
+
+    if (!feof(in))
+    {
+        printf("We should have EOF!\n");
+    }
+
+    fclose(in);
+    return 0;
+}