Reduce comparisons of size_t and int

`Const::size()` returns int, so change iterators that use it to `auto` instead of `size_t`.
For cases where size is being explicitly cast to `int`, use the wrapper that we already have instead: `Yosys::GetSize()`.

kernel/rtlil.cc

@@ -380,7 +380,7 @@ int RTLIL::Const::as_int(bool is_signed) const
 	return ret;
 }
 
-size_t RTLIL::Const::get_min_size(bool is_signed) const
+int RTLIL::Const::get_min_size(bool is_signed) const
 {
 	if (empty()) return 0;
 
@@ -391,7 +391,7 @@ size_t RTLIL::Const::get_min_size(bool is_signed) const
 	else
 		leading_bit = RTLIL::State::S0;
 
-	size_t idx = size();
+	auto idx = size();
 	while (idx > 0 && (*this)[idx -1] == leading_bit) {
 		idx--;
 	}
@@ -406,22 +406,22 @@ size_t RTLIL::Const::get_min_size(bool is_signed) const
 
 void RTLIL::Const::compress(bool is_signed)
 {
-	size_t idx = get_min_size(is_signed);
+	auto idx = get_min_size(is_signed);
 	bits().erase(bits().begin() + idx, bits().end());
 }
 
 std::optional<int> RTLIL::Const::as_int_compress(bool is_signed) const
 {
-	size_t size = get_min_size(is_signed);
+	auto size = get_min_size(is_signed);
 	if(size == 0 || size > 32)
 		return std::nullopt;
 
 	int32_t ret = 0;
-	for (size_t i = 0; i < size && i < 32; i++)
+	for (auto i = 0; i < size && i < 32; i++)
 		if ((*this)[i] == State::S1)
 			ret |= 1 << i;
 	if (is_signed && (*this)[size-1] == State::S1)
-		for (size_t i = size; i < 32; i++)
+		for (auto i = size; i < 32; i++)
 			ret |= 1 << i;
 	return ret;
 }