scudo: Simplify getClassIdBySize() logic. NFCI.

[pcc]

Summary

By subtracting 1 from Size at the beginning we can simplify the
subsequent calculations. This also saves 4 instructions on aarch64
and 9 instructions on x86_64, but seems to be perf neutral.

[merge_guards_bot]

Unit tests: pass. 62433 tests passed, 0 failed and 845 were skipped.

clang-tidy: pass.

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

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compiler-rt/lib/scudo/standalone/size_class_map.h

static uptr getSizeByClassId(uptr ClassId) {
     const uptr T = MidSize << (ClassId >> S);
     return T + (T >> S) * (ClassId & M);
   }
 
   static uptr getClassIdBySize(uptr Size) {
     DCHECK_LE(Size, MaxSize);
     if (Size <= MidSize)
       return (Size + MinSize - 1) >> MinSizeLog;
+    Size -= 1;
     const uptr L = getMostSignificantSetBitIndex(Size);
-    const uptr HBits = (Size >> (L - S)) & M;
-    const uptr LBits = Size & ((1UL << (L - S)) - 1);
-    const uptr L1 = L - MidSizeLog;
-    return MidClass + (L1 << S) + HBits + (LBits > 0);
+    const uptr LBits = (Size >> (L - S)) - (1 << S);
+    const uptr HBits = (L - MidSizeLog) << S;
+    return MidClass + 1 + HBits + LBits;
   }
 
   static u32 getMaxCachedHint(uptr Size) {
     DCHECK_LE(Size, MaxSize);
     DCHECK_NE(Size, 0);
     u32 N;
     // Force a 32-bit division if the template parameters allow for it.
     if (MaxBytesCachedLog > 31 || MaxSizeLog > 31)