[PowerPC] PR19796: Also match ISD::TargetConstant in isIntS16Immediate

The SplitIndexingFromLoad changes exposed a latent isel bug in the PowerPC64
backend.  We matched an immediate offset with STWX8 even though it only
supports register offset.

The culprit is the complex-pattern predicate, SelectAddrIdx, which decides
that if the offset is not ISD::Constant it must be a register.

Many thanks to Bill Schmidt for testing this.