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/HexagonInstrInfo.cpp b/llvm/lib/Target/Hexagon/HexagonInstrInfo.cpp
index 0379451..66e07c6 100644
--- a/llvm/lib/Target/Hexagon/HexagonInstrInfo.cpp
+++ b/llvm/lib/Target/Hexagon/HexagonInstrInfo.cpp
@@ -1254,7 +1254,7 @@
       const MachineOperand &Op1 = MI.getOperand(1);
       const MachineOperand &Op2 = MI.getOperand(2);
       const MachineOperand &Op3 = MI.getOperand(3);
-      LivePhysRegs LiveAtMI(&HRI);
+      LivePhysRegs LiveAtMI(HRI);
       getLiveRegsAt(LiveAtMI, MI);
       bool IsDestLive = !LiveAtMI.available(MRI, Op0.getReg());
       if (Op0.getReg() != Op2.getReg()) {
@@ -1283,7 +1283,7 @@
       MachineOperand &Op1 = MI.getOperand(1);
       MachineOperand &Op2 = MI.getOperand(2);
       MachineOperand &Op3 = MI.getOperand(3);
-      LivePhysRegs LiveAtMI(&HRI);
+      LivePhysRegs LiveAtMI(HRI);
       getLiveRegsAt(LiveAtMI, MI);
       bool IsDestLive = !LiveAtMI.available(MRI, Op0.getReg());