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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226188 91177308-0d34-0410-b5e6-96231b3b80d8