R600/SI: Determine target-specific encoding of READLANE and WRITELANE early v2

These are VOP2 on SI and VOP3 on VI, and their pseudos are neither, which can
be a problem. In order to make isVOP2 and isVOP3 queries behave as expected,
the encoding must be determined first.

This doesn't fix any known issue, but better safe than sorry.

v2: add and use getMCOpcodeFromPseudo

Tested-by: Michel Dänzer <michel.daenzer@amd.com>
llvm-svn: 227987