[LV] Remove `LoopVectorizationCostModel::useEmulatedMaskMemRefHack()`

D43208 extracted `useEmulatedMaskMemRefHack()` from legality into cost model.
What it essentially does is prevents scalarized vectorization of masked memory operations:
```
  // TODO: Cost model for emulated masked load/store is completely
  // broken. This hack guides the cost model to use an artificially
  // high enough value to practically disable vectorization with such
  // operations, except where previously deployed legality hack allowed
  // using very low cost values. This is to avoid regressions coming simply
  // from moving "masked load/store" check from legality to cost model.
  // Masked Load/Gather emulation was previously never allowed.
  // Limited number of Masked Store/Scatter emulation was allowed.
```

While i don't really understand about what specifically `is completely broken`
was talking about, i believe that at least on X86 with AVX2-or-later,
this is no longer true. (or at least, i would like to know what is still broken).
So i would like to follow suit after D111460, and like wise disable that hack for AVX2+.

But since this was added for X86 specifically, let's just instead completely remove this hack.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D114779

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