Simplify hllDenseRegHisto()

In HyperLogLog, every register has 6 bits and every 8 registers can be
processed with same logic. Therefore, the code for handling 16 registers
can be simplified to only handle 8.

src/hyperloglog.c

@@ -504,10 +504,9 @@ void hllDenseRegHisto(uint8_t *registers, int* reghisto) {
      * we take a faster path with unrolled loops. */
     if (HLL_REGISTERS == 16384 && HLL_BITS == 6) {
         uint8_t *r = registers;
-        unsigned long r0, r1, r2, r3, r4, r5, r6, r7, r8, r9,
-                      r10, r11, r12, r13, r14, r15;
-        for (j = 0; j < 1024; j++) {
-            /* Handle 16 registers per iteration. */
+        unsigned long r0, r1, r2, r3, r4, r5, r6, r7;
+        for (j = 0; j < 2048; j++) {
+            /* Handle 8 registers per iteration. */
             r0 = r[0] & 63;
             r1 = (r[0] >> 6 | r[1] << 2) & 63;
             r2 = (r[1] >> 4 | r[2] << 4) & 63;
@@ -516,14 +515,6 @@ void hllDenseRegHisto(uint8_t *registers, int* reghisto) {
             r5 = (r[3] >> 6 | r[4] << 2) & 63;
             r6 = (r[4] >> 4 | r[5] << 4) & 63;
             r7 = (r[5] >> 2) & 63;
-            r8 = r[6] & 63;
-            r9 = (r[6] >> 6 | r[7] << 2) & 63;
-            r10 = (r[7] >> 4 | r[8] << 4) & 63;
-            r11 = (r[8] >> 2) & 63;
-            r12 = r[9] & 63;
-            r13 = (r[9] >> 6 | r[10] << 2) & 63;
-            r14 = (r[10] >> 4 | r[11] << 4) & 63;
-            r15 = (r[11] >> 2) & 63;
 
             reghisto[r0]++;
             reghisto[r1]++;
@@ -533,16 +524,8 @@ void hllDenseRegHisto(uint8_t *registers, int* reghisto) {
             reghisto[r5]++;
             reghisto[r6]++;
             reghisto[r7]++;
-            reghisto[r8]++;
-            reghisto[r9]++;
-            reghisto[r10]++;
-            reghisto[r11]++;
-            reghisto[r12]++;
-            reghisto[r13]++;
-            reghisto[r14]++;
-            reghisto[r15]++;
-
-            r += 12;
+
+            r += 6;
         }
     } else {
         for(j = 0; j < HLL_REGISTERS; j++) {