RISC-V: Fix lui argument parsing.

RISC-V: Fix lui argument parsing.

This fixes a bug reported on the riscv.org sw-dev mailing list.  This
rejects "lui x1,symbol", as a symbol should only be accepted here when
used inside %hi().  Without the fix, this gets assembled as "lui x1,0"
with no relocation which is clearly wrong.

	gas/
	* config/tc-riscv.c (riscv_ip) <'u'>: Move O_constant check inside if
	statement.  Delete O_symbol and O_constant check after if statement.
	* testsuite/gas/riscv/auipc-parsing.s: Test lui with missing %hi.
	* testsuite/gas/riscv/auipc-parsing.l: Update.