LivePhysRegs: Rework constructor + documentation; NFC

- Take reference instead of pointer to a TRI that cannot be nullptr.
- Improve documentation comments.

llvm-svn: 304038
diff --git a/llvm/lib/Target/Hexagon/HexagonFrameLowering.cpp b/llvm/lib/Target/Hexagon/HexagonFrameLowering.cpp
index a04aca4..25018b9 100644
--- a/llvm/lib/Target/Hexagon/HexagonFrameLowering.cpp
+++ b/llvm/lib/Target/Hexagon/HexagonFrameLowering.cpp
@@ -1657,7 +1657,7 @@
   // defined. From the point of view of the liveness tracking, it is ok to
   // store it as a whole, but if we break it up we may end up storing a
   // register that is entirely undefined.
-  LivePhysRegs LPR(&HRI);
+  LivePhysRegs LPR(HRI);
   LPR.addLiveIns(B);
   SmallVector<std::pair<unsigned, const MachineOperand*>,2> Clobbers;
   for (auto R = B.begin(); R != It; ++R) {