[AMDGPU][MC] Added missing VOP3P opcodes

Added support of the following opcodes:
  v_pk_sub_u16
  v_pk_mad_i16
  v_pk_mad_u16

See Bug 33593: https://bugs.llvm.org//show_bug.cgi?id=33593

Reviewers: vpykhtin, artem.tamazov, arsenm

Differential Revision: https://reviews.llvm.org/D34890

llvm-svn: 308281