R600/SI: Trivial instruction definition corrections for VI (v2)

- V_MAC_LEGACY_F32 exists on VI, but it's VOP3-only.

- Define CVT_PK opcodes which are different between SI and VI. These are
  unused. The idea is to define all chip differences.

v2: keep V_MUL_LO_U32

Tested-by: Michel Dänzer <michel.daenzer@amd.com>
llvm-svn: 227988