R600/SI: Don't shrink instructions whose e32 encoding doesn't exist

v2: modify hasVALU32BitEncoding instead
v3: - add pseudoToMCOpcode helper to AMDGPUInstInfo, which is used by both
      hasVALU32BitEncoding and AMDGPUMCInstLower::lower
    - report an error if a pseudo can't be lowered
llvm-svn: 226188