MIPS sign extend
We expect a 32-bit number in Read Data 2 (RD2) register.
To do so, we need to extend our 16-bit input.
We want to preserve the sign, so we need to fill the remaining bits with the MSB.
E.g.
50 = 0000 0000 0011 0010
-50 = 1111 1111 1100 1110
-- Sign extend to 32 bits
-50 = 1111 1111 1111 1111
= 1111 1111 1100 1110
50 = 0000 0000 0000 0000
= 0000 0000 0011 0010