[RISCV] Use ZERO_EXTEND instead of ANY_EXTEND when promoting i32 RISCVISD::SHFL. NFC

We know the shift amount is a constant with bit 31 clear. anyext
of constant will be either zext or sext which will produce the
same result here. But we really shouldn't rely on that. It would
be valid to put a random number in the upper bits. Our isel patterns
expect the upper bits to be 0 so we should ask for it explicitly.