Factor code from equal_at_ignore_case for use in index_ignore_case

MVM_string_equal_at_ignore_case_INTERNAL_loop is the internal loop that
will be used be MVM_string_index_ignore_case. This will allow sharing
the code between MVM_string_index_ignore_case and
MVM_string_equal_at_ignore_case and prevent the index function from getting
too messy.

src/strings/ops.c

@@ -585,6 +585,44 @@ MVMint64 MVM_string_equal_at(MVMThreadContext *tc, MVMString *a, MVMString *b, M
         return 0;
     return MVM_string_substrings_equal_nocheck(tc, a, offset, bgraphs, b, 0);
 }
+MVM_STATIC_INLINE MVMint32 string_equal_at_ignore_case_INTERNAL_loop(MVMThreadContext *tc, MVMString *haystack, MVMString *needle_fc, MVMint64 h_start, MVMint64 h_graphs, MVMint64 n_graphs) {
+    MVMuint32 h_fc_cps;
+    /* An additional needle offset which is used only when codepoints expand
+     * when casefolded. The offset is the number of additional codepoints that
+     * have been seen so haystack and needle stay aligned */
+    MVMint64 n_offset = 0;
+    MVMint64 i, j;
+    MVMGrapheme32 h_g, n_g;
+    for (i = 0; i + h_start < h_graphs && i + n_offset < n_graphs; i++) {
+        const MVMCodepoint* h_result_cps;
+        h_g = MVM_string_get_grapheme_at_nocheck(tc, haystack, h_start + i);
+        if (h_g >= 0 ) {
+            /* For codeponits we can get the case change directly */
+            h_fc_cps = MVM_unicode_get_case_change(tc, h_g, MVM_unicode_case_change_type_fold, &h_result_cps);
+        }
+        else {
+            /* Synthetics must use this function */
+            h_fc_cps = MVM_nfg_get_case_change(tc, h_g, MVM_unicode_case_change_type_fold, (MVMGrapheme32**) &h_result_cps);
+        }
+        /* If we get 0 for the number that means the cp doesn't change when casefolded */
+        if (h_fc_cps == 0) {
+            n_g = MVM_string_get_grapheme_at_nocheck(tc, needle_fc, i + n_offset);
+            if (h_g != n_g)
+                return 0;
+        }
+        else if (h_fc_cps >= 1) {
+            for (j = 0; j < h_fc_cps; j++) {
+                n_g = MVM_string_get_grapheme_at_nocheck(tc, needle_fc, i + n_offset);
+                h_g = h_result_cps[j];
+                if (h_g != n_g)
+                    return 0;
+                n_offset++;
+            }
+            n_offset--;
+        }
+    }
+    return 1;
+}
 /* Checks if needle exists at the offset, but ignores case
  * Sometimes there is a difference in length of a string before and after foldcase,
  * because of this we must compare this differently than just foldcasing both