[PowerPC] Remove allow-deprecated-dag-overlap and fix broken tests

Summary:
This is found during review of https://reviews.llvm.org/D67088.

CHECK-DAG is non-overlapping after https://reviews.llvm.org/D47106.
-allow-deprecated-dag-overlap was introduced to temporary accept old
behavior.

But it actually hide some broken tests, eg: `test/CodeGen/PowerPC/swaps-le-1.ll`
The codegen has changed, but the CHECK-DAG still PASS due to allowing `overlap`.

This patch remove the deprecated options, and fix the broken tests.

Reviewers: #powerpc, hfinkel, nemanjai, steven.zhang, shchenz

Reviewed By: shchenz

Subscribers: shchenz, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69733