[RISC-V] Fix false positive from Wuninitialized

[RISC-V] Fix false positive from Wuninitialized

As Mark and I independently tripped, there's a Wuninitialized issue in the
RISC-V backend.  While *I* know the value would always be properly initialized,
it'd be somewhat painful to either eliminate the infeasible paths or do deep
enough analysis to suppress the false positive.

So this initializes OUTPUT and verifies it's got a reasonable value before
using it for the final copy into operands[0].

Bootstrapped on the BPI (regression testing still has ~12hrs to go).

gcc/
	* config/riscv/riscv.cc (synthesize_ior_xor): Initialize OUTPUT and
	verify it's non-null before emitting the final copy insn.