[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