[RISCV] Update comments in RISCVMatInt to reflect we don't always use ADDIW after LUI now. NFC (#159829)
The simm32 base case only uses lui+addiw when necessary after
3d2650bdeb8409563d917d8eef70b906323524ef
The worst case 8 instruction sequence doesn't leave a full 32 bits for
the LUI+ADDI(W) after the 3 12-bit ADDI and SLLI pairs are created. So
we will never generate LUI+ADDIW in the worst case sequence.