[X86] Split masked integer vector stores into vXi32/vXi64 variants (PR45975). NFC
This effectively splits the scheduling WriteVecMaskedStore(Y) classes
into four different classes (one per each variant).
The new VecMaskedStore scheduling classes are now correctly marked as
'unsupported' by the bdver2 and btver2 models.
No functional change intended.
Reviewed By: RKSimon
Differential Revision: https://reviews.llvm.org/D80201