[PowerPC] Change std::sort to llvm::sort in response to r327219

[mgrang]

Summary

r327219 added wrappers to std::sort which randomly shuffle the container before sorting.
This will help in uncovering non-determinism caused due to undefined sorting
order of objects having the same key.

To make use of that infrastructure we need to invoke llvm::sort instead of std::sort.

Note: This patch is one of a series of patches to replace *all* std::sort to llvm::sort.
Refer the comments section in D44363 for a list of all the required patches.

[RKSimon]

LGTM

lib/Target/PowerPC/PPCISelDAGToDAG.cpp

void collectValueRotInfo() {
     }
 
     // Now that we've collected the various ValueRotInfo instances, we need to
     // sort them.
     ValueRotsVec.clear();
     for (auto &I : ValueRots) {
       ValueRotsVec.push_back(I.second);
     }
-    std::sort(ValueRotsVec.begin(), ValueRotsVec.end());
+    llvm::sort(ValueRotsVec.begin(), ValueRotsVec.end());
   }
 
   // In 64-bit mode, rlwinm and friends have a rotation operator that
   // replicates the low-order 32 bits into the high-order 32-bits. The mask
   // indices of these instructions can only be in the lower 32 bits, so they
   // can only represent some 64-bit bit groups. However, when they can be used,
   // the 32-bit replication can be used to represent, as a single bit group,
   // otherwise separate bit groups. We'll convert to replicated-32-bit bit