[sanitizer] SizeClassMap minor improvement/correctness changes

[cryptoad]

Summary

In ClassID, make sure we use an unsigned as based for the lbits shift.
The previous code resulted in spurious sign extensions like for x64:

add     esi, 0FFFFFFFFh
movsxd  rcx, esi
and     rcx, r15

The code with the U added is:

add     esi, 0FFFFFFFFh
and     rsi, r15

And for MaxCachedHint, use a 32-bit division instead of 64-bit, which is
faster (https://lemire.me/blog/2017/11/16/fast-exact-integer-divisions-using-floating-point-operations/)
and already used in other parts of the code (64-bit GetChunkIdx, 32-bit
GetMetaData enforce 32-bit divisions)

Not major performance gains by any mean, but they don't hurt.

compiler-rt/trunk/lib/sanitizer_common/sanitizer_allocator_size_class_map.h

public:
 
   static uptr ClassID(uptr size) {
     if (UNLIKELY(size > kMaxSize))
       return 0;
     if (size <= kMidSize)
       return (size + kMinSize - 1) >> kMinSizeLog;
     uptr l = MostSignificantSetBitIndex(size);
     uptr hbits = (size >> (l - S)) & M;
-    uptr lbits = size & ((1 << (l - S)) - 1);
+    uptr lbits = size & ((1U << (l - S)) - 1);
     uptr l1 = l - kMidSizeLog;
     return kMidClass + (l1 << S) + hbits + (lbits > 0);
   }
 
   static uptr MaxCachedHint(uptr class_id) {
     // Estimate the result for kBatchClassID because this class does not know
     // the exact size of TransferBatch. We need to cache fewer batches than user
     // chunks, so this number can be small.
     if (UNLIKELY(class_id == kBatchClassID))
       return 16;
     if (UNLIKELY(class_id == 0))
       return 0;
-    uptr n = (1UL << kMaxBytesCachedLog) / Size(class_id);
+    const uptr n =
+        (1U << kMaxBytesCachedLog) / static_cast<u32>(Size(class_id));
     return Max<uptr>(1, Min(kMaxNumCachedHint, n));
   }
 
   static void Print() {
     uptr prev_s = 0;
     uptr total_cached = 0;
     for (uptr i = 0; i < kNumClasses; i++) {
       uptr s = Size(i);