Use snek_code_prev_insn() instead of snek_compile_prev

Be more consistent about using accessor functions.

Signed-off-by: Keith Packard <keithp@keithp.com>

snek-gram.ll

@@ -38,7 +38,7 @@ command		: @{ snek_print_val = snek_interactive; }@ stat
 	 		}@
 		  OP opt-formals CP COLON suite
 			@{
-				if (snek_compile[snek_compile_prev] == snek_op_return)
+				if (snek_compile[snek_code_prev_insn()] == snek_op_return)
 					snek_code_delete_prev();
 				else
 					snek_code_add_op(snek_op_null);
@@ -215,7 +215,7 @@ if-expr		: expr COLON
 				snek_code_add_op_offset(snek_op_branch_false, 0);
 
 				/* push 0 - if_expr_off */
-				value_push_offset(snek_compile_prev);
+				value_push_offset(snek_code_prev_insn());
 			}@
 		;
 while-stat	:
@@ -227,13 +227,13 @@ while-stat	:
 			@{
 				snek_code_add_op_offset(snek_op_branch_false, 0);
 				/* push 1 - while_off */
-				value_push_offset(snek_compile_prev);
+				value_push_offset(snek_code_prev_insn());
 			}@
 		  suite
 			@{
 				/* push 2 - loop_end_off */
 				snek_code_add_op_offset(snek_op_branch, 0);
-				value_push_offset(snek_compile_prev);
+				value_push_offset(snek_code_prev_insn());
 				/* push 3 - while_else_stat_off */
 				value_push_offset(snek_code_current());
 			}@
@@ -251,7 +251,7 @@ for-stat	: FOR NAME
 			@{
 				snek_code_add_op_offset(snek_op_branch, 0);
 				/* push 2 - loop_end_off */
-				value_push_offset(snek_compile_prev);
+				value_push_offset(snek_code_prev_insn());
 				/* push 3 - while_else_stat_off */
 				value_push_offset(snek_code_current());
 				for_depth--;
@@ -268,9 +268,9 @@ for-params	: RANGE OP opt-actuals CP COLON
 				snek_code_add_in_range(id, num, for_depth);
 			for_push_prevs:
 				/* push 0 - for_off */
-				value_push_offset(snek_compile_prev);
+				value_push_offset(snek_code_prev_insn());
 				/* push 1 - top_off */
-				value_push_offset(snek_compile_prev);
+				value_push_offset(snek_code_prev_insn());
 				for_depth++;
 			}@
 		| expr COLON