[lld][macho] Use reloc length correctly in hash computation (#165287)

`Reloc::length` actually encodes the log2 of the length. Thanks @int3
for pointing this out in
#160894 (comment).

This code computes hashes of relocations. With the correct length, the
hashes should more accurately represent the relocation. In my testing of
some large binaries, the compressed size change is negligable.

Author: ellishg

lld/MachO/BPSectionOrderer.cpp

@@ -61,12 +61,13 @@ struct BPOrdererMachO : lld::BPOrderer<BPOrdererMachO> {
 
     // Calculate relocation hashes
     for (const auto &r : sec.relocs) {
-      if (r.length == 0 || r.referent.isNull() || r.offset >= data.size())
+      uint32_t relocLength = 1 << r.length;
+      if (r.referent.isNull() || r.offset + relocLength > data.size())
         continue;
 
       uint64_t relocHash = getRelocHash(r, sectionToIdx);
       uint32_t start = (r.offset < windowSize) ? 0 : r.offset - windowSize + 1;
-      for (uint32_t i = start; i < r.offset + r.length; i++) {
+      for (uint32_t i = start; i < r.offset + relocLength; i++) {
         auto window = data.drop_front(i).take_front(windowSize);
         hashes.push_back(xxh3_64bits(window) ^ relocHash);
       }