Add RTLIL::Const::ext[su](), fix RTLIL::SigSpec::extend_u0 for 0-size signals

Signed-off-by: Clifford Wolf <clifford@clifford.at>

kernel/rtlil.cc

@@ -3237,7 +3237,7 @@ void RTLIL::SigSpec::extend_u0(int width, bool is_signed)
 		remove(width, width_ - width);
 
 	if (width_ < width) {
-		RTLIL::SigBit padding = width_ > 0 ? (*this)[width_ - 1] : RTLIL::State::S0;
+		RTLIL::SigBit padding = width_ > 0 ? (*this)[width_ - 1] : RTLIL::State::Sx;
 		if (!is_signed)
 			padding = RTLIL::State::S0;
 		while (width_ < width)

kernel/rtlil.h

@@ -546,6 +546,14 @@ struct RTLIL::Const
 		return ret;
 	}
 
+	void extu(int width) {
+		bits.resize(width, RTLIL::State::S0);
+	}
+
+	void exts(int width) {
+		bits.resize(width, bits.empty() ? RTLIL::State::Sx : bits.back());
+	}
+
 	inline unsigned int hash() const {
 		unsigned int h = mkhash_init;
 		for (auto b : bits)