_BitScan{Reverse,Forward} add check for undefined

[bimbashrestha]

#1951

I've managed to reproduce this error on my windows vm, visual 2019 with:

#include <iostream>
#include <cassert>

using namespace std;

int main()
{
  unsigned long index = 0;
  _BitScanReverse64(&index, 0x0ull);

  cout << index << endl;

  assert(index == 0);

  return 0;
}

Silesia entire file benchmarks:

level	zstd-clang (before)	zstd-clang (after)	zstd-gcc (before)	zstd-gcc (after)
1	440 mb/s	440 mb/s (-)	454 mb/s	454 mb/s (-)
2	334 mb/s	337 mb/s (-)	334 mb/s	336 mb/s (-)
3	239 mb/s	243 mb/s (+1%)	241 mb/s	242 mb/s (-)
4	233 mb/s	230 mb/s (-1%)	226 mb/s	227 mb/s (-)
5	126 mb/s	128 mb/s (-)	126 mb/s	127 mb/s (-)
6	101 mb/s	101 mb/s (-)	102 mb/s	102 mb/s (-)
7	68 mb/s	67 mb/s (-)	72 mb/s	72 mb/s (-)
8	53 mb/s	54 mb/s (-)	58 mb/s	58 mb/s (-)
9	38 mb/s	38 mb/s (-)	42 mb/s	42 mb/s (-)
10	25 mb/s	25 mb/s (-)	27 mb/s	27 mb/s (-)
11	19 mb/s	19 mb/s (-)	19 mb/s	19 mb/s (-)
12	13 mb/s	14 mb/s (+4%)	13 mb/s	13 mb/s (-)
13	11.0 mb/s	10.95 mb/s (-)	10.99 mb/s	10.95 mb/s (-)
14	8.49 mb/s	8.44 mb/s (-)	8.16 mb/s	8.11 mb/s (-)
15	6.40 mb/s	6.34 mb/s (-)	6.24 mb/s	6.22 mb/s (-)
16	4.94 mb/s	4.92 mb/s (-)	5.15 mb/s	5.25 mb/s (-)
17	3.97 mb/s	3.95 mb/s (-)	4.06 mb/s	4.07 mb/s (-)
18	3.20 mb/s	3.20 mb/s (-)	3.31 mb/s	3.33 mb/s (-)
19	2.64 mb/s	2.66 mb/s (-)	2.77 mb/s	2.80 mb/s (-)

[terrelln]

Except for the typo, the change looks good to me, just need the benchmarks.

[bimbashrestha]

Posted the benchmarks above. This seems pretty neutral to me for both gcc and clang.