[NFC] Add parentheses in MathExtra.h

The code used to cause a warning:
  llvm/include/llvm/Support/MathExtras.h:751:39: warning: suggest parentheses around ‘-’ in operand of ‘&’ [-Wparentheses]
    751 |   assert(Align != 0 && (Align & Align - 1) == 0 &&
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