[RegisterCoalescer] Fixed inconsistent followCopyChain with subreg

[tpr]

Summary

The behavior of followCopyChain with a subreg depends on the order in
which subranges appear in a live interval, which is bad.

This commit fixes that, and allows the copy chain to continue only if
all matching subranges that are not undefined take us to the same def.

I don't have a test for this; the reproducer I had on my branch with
various other local changes does not reproduce the problem on upstream
llvm. Also that reproducer was an ll test; attempting to convert it to a
mir test made the subranges appear in a different order and hid the
problem.

However I would argue that the old behavior was obviously wrong
and needs fixing.

Change-Id: Iee7936ef305918f3b498ac432e2cf651ae5cc2df

[kparzysz]

This looks reasonable.

llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp

while (!VNI->isPHIDef()) {
 
     const LiveInterval &LI = LIS->getInterval(SrcReg);
     const VNInfo *ValueIn;
     // No subrange involved.
     if (!SubRangeJoin || !LI.hasSubRanges()) {
       LiveQueryResult LRQ = LI.Query(Def);
       ValueIn = LRQ.valueIn();
     } else {
-      // Query subranges. Pick the first matching one.
+      // Query subranges. Ensure that all matching ones take us to the same def
+      // (allowing some of them to be undef).
       ValueIn = nullptr;
       for (const LiveInterval::SubRange &S : LI.subranges()) {
         // Transform lanemask to a mask in the joined live interval.
         LaneBitmask SMask = TRI->composeSubRegIndexLaneMask(SubIdx, S.LaneMask);
         if ((SMask & LaneMask).none())
           continue;
         LiveQueryResult LRQ = S.Query(Def);
+        if (!ValueIn) {
           ValueIn = LRQ.valueIn();
-        break;
+          continue;
+        }
+        if (LRQ.valueIn() && ValueIn != LRQ.valueIn())
+          return std::make_pair(VNI, TrackReg);
       }
     }
     if (ValueIn == nullptr) {
       // Reaching an undefined value is legitimate, for example:
       //
       // 1   undef %0.sub1 = ...  ;; %0.sub0 == undef
       // 2   %1 = COPY %0         ;; %1 is defined here.
       // 3   %0 = COPY %1         ;; Now %0.sub0 has a definition,