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/PowerPC/PPCExpandISEL.cpp b/llvm/lib/Target/PowerPC/PPCExpandISEL.cpp
index ebd414b..41e3190 100644
--- a/llvm/lib/Target/PowerPC/PPCExpandISEL.cpp
+++ b/llvm/lib/Target/PowerPC/PPCExpandISEL.cpp
@@ -339,7 +339,7 @@
     // Note: Cannot use stepBackward instead since we are using the Reg
     // liveness state at the end of MBB (liveOut of MBB) as the liveIn for
     // NewSuccessor. Otherwise, will cause cyclic dependence.
-    LivePhysRegs LPR(MF->getSubtarget<PPCSubtarget>().getRegisterInfo());
+    LivePhysRegs LPR(*MF->getSubtarget<PPCSubtarget>().getRegisterInfo());
     SmallVector<std::pair<unsigned, const MachineOperand *>, 2> Clobbers;
     for (MachineInstr &MI : *MBB)
       LPR.stepForward(MI, Clobbers);