[X86] AVX512: Simplify logic in isCDisp8

It was computing the VL/n case as:
  MemObjSize = VectorByteSize / ElemByteSize / Divider * ElemByteSize

ElemByteSize not only falls out but VectorByteSize/Divider now actually
matches the definition of VL/n.

Also some formatting fixes.
[X86] AVX512: Improve readability of isCDisp8

No functional change.  As I was trying to understand this function, I found
that variables were reused with confusing names and the broadcast case was a
bit too implicit.  Hopefully, this is an improvement.