[RISCV] Remove performANY_EXTENDCombine.

This doesn't appear to be needed any more. I did some inspecting
of the gcc torture suite and SPEC2006 with this removed and didn't
find any meaningful changes.

I think we're more aggressive about forming ADDIW now using
sign_extend_inreg during type legalization and hasAllWUsers in isel.
This probably helps catch the cases this helped with before.