[ARM] VFPv2 only supports 16 D registers.

r361845 changed the way we handle "D16" vs. "D32" targets; there used to
be a negative "d16" which removed instructions from the instruction set,
and now there's a "d32" feature which adds instructions to the
instruction set.  This is good, but there was an oversight in the
implementation: the behavior of VFPv2 was changed.  In particular, the
"vfp2" feature was changed to imply "d32". This is wrong: VFPv2 only
supports 16 D registers.

In practice, this means if you specify -mfpu=vfpv2, the compiler will
generate illegal instructions.

This patch gets rid of "vfp2d16" and "vfp2d16sp", and fixes "vfp2" and
"vfp2sp" so they don't imply "d32".

Differential Revision: https://reviews.llvm.org/D67375

llvm-svn: 372186