[X86] Move getMaskNode to avoid unnecessary forward declarations. (#186815)

I've also improved the assertions on the source / bool mask types to
catch bad use cases.

Cleanup pre-work to allow the i512 codegen to eventually use getMaskNode
instead of manual bool mask creations

GitOrigin-RevId: 1c2a6504da66c2762cc983b6b61bf0fccbf4c09c
1 file changed