Finally fix one of the oldest FIXMEs in the PowerPC backend: correctly

Finally fix one of the oldest FIXMEs in the PowerPC backend: correctly
flag rotate left word immediate then mask insert (rlwimi) as a two-address
instruction, and update the ISel usage of the instruction accordingly.

This will allow us to properly schedule rlwimi, and use it to efficiently
codegen bitfield operations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@17068 91177308-0d34-0410-b5e6-96231b3b80d8