R600/SI: Don't generate non-existent LSHL, LSHR, ASHR B32 variants on VI

R600/SI: Don't generate non-existent LSHL, LSHR, ASHR B32 variants on VI

This can happen when a REV instruction is commuted.

The trick is not to define the _vi versions of instructions, which has these
consequences:
- code generation will always fail if a pseudo cannot be lowered
  (very useful to catch bugs where an unsupported instruction somehow makes
   it to the printer)
- ability to query if a pseudo can be lowered, which is done in commuteOpcode
  to prevent REV from commuting to non-REV on VI

Tested-by: Michel Dänzer <michel.daenzer@amd.com>
llvm-svn: 227990