GlobalISel: permit unused vregs without a register-class after ISel.

This can happen if earlier combining has removed all uses of some VReg, which
is fine and shouldn't flag an error.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293537 91177308-0d34-0410-b5e6-96231b3b80d8