compiler: use correct size and comparison in index value overflow check

This has apparently been wrong since I introduced the code ten years ago.

Fixes PR go/114500

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/574835

Author: ianlancetaylor

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-3f597287b6b858794dabdfe1bf83b386aad18102
+98e92493db2ab7857a5934a950a830fc1f95a4e5
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -18790,7 +18790,7 @@ Composite_literal_expression::lower_array(Type* type)
 
 	  Named_type* ntype = Type::lookup_integer_type("int");
 	  Integer_type* inttype = ntype->integer_type();
-	  if (sizeof(index) <= static_cast<size_t>(inttype->bits() * 8)
+	  if (sizeof(index) >= static_cast<size_t>(inttype->bits() / 8)
 	      && index >> (inttype->bits() - 1) != 0)
 	    {
 	      go_error_at(index_expr->location(), "index value overflow");