[DAGCombine][X86][AArch64] Masked merge unfolding: vector edition.

Summary:
This **appears** to be the last missing piece for the masked merge pattern handling in the backend.

This is [[ https://bugs.llvm.org/show_bug.cgi?id=37104 | PR37104 ]].

[[ https://bugs.llvm.org/show_bug.cgi?id=6773 | PR6773 ]] will introduce an IR canonicalization that is likely bad for the end assembly.
Previously, `andps`+`andnps` / `bsl` would be generated. (see `@out`)
Now, they would no longer be generated  (see `@in`), and we need to make sure that they are generated.

Differential Revision: https://reviews.llvm.org/D46528

llvm-svn: 332904