[sanitizer] SizeClassMap minor improvement/correctness changes

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.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: kubamracek, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D42916

llvm-svn: 324263

compiler-rt/lib/sanitizer_common/sanitizer_allocator_size_class_map.h

@@ -163,7 +163,7 @@ class SizeClassMap {
       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);
   }
@@ -176,7 +176,8 @@ class SizeClassMap {
       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));
   }