[xlscc] Variable set_slc, or at least should work with loop counter

[spurserh]

void reverse_nibbles(sai16 &x) {
#pragma hls_unroll yes
for(int b=0;b<16;b+=4) {
x.set_slc(b, x.slc<4>(12-b));
}
}

[cdleary]

Closing all the bugs filed against the old xlscc, if this same issue is encountered with the Clang-based FE we'll file a new bug / make a new test case.