[MSan] Remove an imposible condition check from CopyOrigin

[stephan.yichao.zhao]

Summary

  uptr beg = d & ~3UL;
  uptr end = (d + size) & ~3UL;

So end is always >= beg unless overflowed.
But overflow should be checked in a different way.

compiler-rt/lib/msan/msan_poisoning.cpp

if (beg < d) {
     if (o) {
       if (__msan_get_track_origins() > 1) o = ChainOrigin(o, stack);
       *(u32 *)MEM_TO_ORIGIN(beg) = o;
     }
     beg += 4;
   }
 
   uptr end = (d + size) & ~3UL;
-  // If both ends fall into the same 4-byte slot, we are done.
-  if (end < beg) return;
 
   // Copy right unaligned origin if that memory is poisoned.
   if (end < d + size) {
     u32 o = GetOriginIfPoisoned((uptr)src + (end - d), (d + size) - end);
     if (o) {
       if (__msan_get_track_origins() > 1) o = ChainOrigin(o, stack);
       *(u32 *)MEM_TO_ORIGIN(end) = o;
     }