tgt-vvp: Fix out-of-bounds compressed assignment to arrays

If the index of an array access is known to be out-of-bounds during
elaboration it is replaced with 'x. In the tgt-vvp backend that is handling
compressed array assignments there is an assert() that triggers if the
index is an undefined immediate.

There is already an existing code path that is capable of handling
out-of-bounds access. Remove the assert and set the index to ULONG_MAX to
trigger taking the out-of-bound access path.

On this out-of-bounds path the write to the array is skipped. But this
leaves the result on the vector stack. Insert the `%pop/vec4` instruction
to make sure it is removed.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

tgt-vvp/stmt_assign.c

@@ -21,6 +21,7 @@
 # include  <string.h>
 # include  <assert.h>
 # include  <stdlib.h>
+# include  <limits.h>
 
 /*
  * These functions handle the blocking assignment. Use the %set
@@ -101,8 +102,10 @@ static void get_vec_from_lval_slice(ivl_lval_t lval, struct vec_slice_info*slice
 	   it to select the word, and pay no further heed to the
 	   expression itself. */
       if (word_ix && number_is_immediate(word_ix, IMM_WID, 0)) {
-	    assert(! number_is_unknown(word_ix));
-	    use_word = get_number_immediate(word_ix);
+	    if (number_is_unknown(word_ix))
+		  use_word = ULONG_MAX; // The largest valid index is ULONG_MAX - 1
+	    else
+		  use_word = get_number_immediate(word_ix);
 	    word_ix = 0;
       }
 
@@ -321,6 +324,7 @@ static void put_vec_to_lval_slice(ivl_lval_t lval, struct vec_slice_info*slice,
 		  clr_word(word_idx);
 	    } else {
 		  fprintf(vvp_out," ; Skip this slice write to v%p [%lu]\n", sig, slice->u_.memory_word_static.use_word);
+		  fprintf(vvp_out,"    %%pop/vec4 1;\n");
 	    }
 	    break;