[9.0 branch][ARM] VFPv2 only supports 16 D registers.

Summary:
Patch for 9.0.1.

Simplified version of r372186/r372187: fix the meaning of the "vfpv2" and "vfpv2sp" features, but keep around the useless "vfp2d16" and "vfp2d16sp" features, to reduce the risk on the release branch.

Fixes https://bugs.llvm.org/show_bug.cgi?id=43365

Reviewers: t.p.northover, tstellar

Reviewed By: t.p.northover

Subscribers: kristof.beyls, hiraditya, kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68675

llvm-svn: 374433