Implement widening and narrowing of all EPI supported types

# Conflicts:
#	llvm/test/CodeGen/RISCV/epi-vp-fptosi.ll
#	llvm/test/CodeGen/RISCV/epi-vp-fptoui.ll
#	llvm/test/CodeGen/RISCV/epi-vp-sitofp.ll
#	llvm/test/CodeGen/RISCV/epi-vp-uitofp.ll