[X86][SelectionDAG] Cleanup and simplify masked_load/masked_store in tablegen....

[X86][SelectionDAG] Cleanup and simplify masked_load/masked_store in tablegen. Use more precise PatFrags for scalar masked load/store.

Rename masked_load/masked_store to masked_ld/masked_st to discourage
their direct use. We need to check truncating/extending and
compressing/expanding before using them. This revealed that
our scalar masked load/store patterns were misusing these.

With those out of the way, renamed masked_load_unaligned and
masked_store_unaligned to remove the "_unaligned". We didn't
check the alignment anyway so the name was somewhat misleading.

Make the aligned versions inherit from masked_load/store instead
from a separate identical version. Merge the 3 different alignments
PatFrags into a single version that uses the VT from the SDNode to
determine the size that the alignment needs to match.

llvm-svn: 364150