Fix %ix/load to work with all index registers.

draw_number_bool64() in tgt-vvp/eval_bool.c was using %ix/load to
load immediate values into registers greater than three. The problem
was that of_IX_LOAD() in vvp/vthread.cc was masking off the upper
bits. This was putting the results in the wrong register. This patch
removes the bit masking from of_IX_LOAD() and updates the %ix/load
documentation.

vvp/opcodes.txt

@@ -342,9 +342,9 @@ bits 4/5/6 just line %ix/get.
 
 This instruction loads an immediate value into the addressed index
 register. The index register holds numeric values, so the <value> is a
-number. The idx value selects the index register, and may be 0, 1, 2
-or 3. This is different from %ix/get, which loads the index register
-from a value in the thread bit vector.
+number. The idx value selects the index register. This is different
+from %ix/get, which loads the index register from a value in the
+thread bit vector.
 
 
 * %ix/add <idx>, <value>

vvp/vthread.cc

@@ -1858,7 +1858,7 @@ bool of_IX_MUL(vthread_t thr, vvp_code_t cp)
 
 bool of_IX_LOAD(vthread_t thr, vvp_code_t cp)
 {
-      thr->words[cp->bit_idx[0] & 3].w_int = cp->number;
+      thr->words[cp->bit_idx[0]].w_int = cp->number;
       return true;
 }