[pruned-liveness] Change PrunedLiveBlocks::getLiveness(...) to return only one entry per live bit.

Previously, we would pan the array with isDead. The thinking is that this would
make it easier to bitmask directly against. In practice, this usage was a minor
use case and doing this lead to a bunch of logic mistakes in forthcoming code.

Author: gottesmm

include/swift/SIL/PrunedLiveness.h

@@ -209,13 +209,6 @@ class PrunedLiveBlocks {
       unsigned actualStartBitNo = startBitNo * 2;
       unsigned actualEndBitNo = endBitNo * 2;
 
-      // NOTE: We pad both before/after with Dead to ensure that we are
-      // returning an array that acts as a bit mask and thus can be directly
-      // compared against other such bitmasks. This invariant is used when
-      // computing boundaries.
-      for (unsigned i = 0; i != startBitNo; ++i) {
-        resultingFoundLiveness.push_back(Dead);
-      }
       for (unsigned i = actualStartBitNo, e = actualEndBitNo; i != e; i += 2) {
         if (!bits[i]) {
           resultingFoundLiveness.push_back(Dead);
@@ -224,9 +217,6 @@ class PrunedLiveBlocks {
 
         resultingFoundLiveness.push_back(bits[i + 1] ? LiveOut : LiveWithin);
       }
-      for (unsigned i = endBitNo, e = size(); i != e; ++i) {
-        resultingFoundLiveness.push_back(Dead);
-      }
     }
 
     void setLiveness(unsigned startBitNo, unsigned endBitNo, IsLive isLive) {
@@ -314,7 +304,7 @@ class PrunedLiveBlocks {
                         SmallVectorImpl<IsLive> &foundLivenessInfo) const {
     auto liveBlockIter = liveBlocks.find(bb);
     if (liveBlockIter == liveBlocks.end()) {
-      for (unsigned i : range(numBitsToTrack)) {
+      for (unsigned i : range(endBitNo - startBitNo)) {
         (void)i;
         foundLivenessInfo.push_back(Dead);
       }