1. Rename nomask as unmasked to keep with the terminology in the spec.
2. Merge UnMaskpolicy and Maskedpolicy arguments into one in RVVBuiltin class.
3. Rename HasAutoDef as HasBuiltinAlias.
4. Move header definition code into one class.
Reviewed By: rogfer01
Differential Revision: https://reviews.llvm.org/D120870