[Doc] Proposal for vector predication

Summary:
Proposal and roadmap towards vector predication in LLVM.
This patch documents that
a) It is recognized that current LLVM is ill-equipped for vector predication.
b) The community is working on a solution.
c) A concrete prototype exists in the VP extension (D57504).

Reviewers: rkruppe, rengolin, cameron.mcinally, SjoerdMeijer, andrew.w.kaylor, craig.topper, sdesmalen, k-ishizaka, lattner, fhahn

Reviewed By: andrew.w.kaylor

Subscribers: rogfer01, merge_guards_bot, simoncook, s.egerton, llvm-commits, efocht

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73889