[RISCV][LegalizeDAG] Expand SETO and SETUO comparisons. Teach LegalizeDAG to...

[RISCV][LegalizeDAG] Expand SETO and SETUO comparisons. Teach LegalizeDAG to expand SETUO expansion when UNE isn't legal.

If SETUNE isn't legal, UO can use the NOT of the SETO expansion.

Removes some complex isel patterns. Most of the test changes are
from using XORI instead of SEQZ.

Differential Revision: https://reviews.llvm.org/D92008