GlobalISel: remove variable_ops from output list.

The instance in the input operand list allows both inputs and outputs,
but the one in (outs) is not treated specially which leads to the
MachineVerifier invoking UB (looking at an invalid MCInstrDesc field).

No functional change except in UBSan builds (maybe, who knows!), where
it fixes the legalize-add.mir test.

llvm-svn: 276872