[MachineFunction] Make comment for TracksLiveness more clearer

[Carrot]

Summary

As discussed in https://lists.llvm.org/pipermail/llvm-dev/2021-April/150225.html, the current comments for TracksLiveness property and isKill flag are confusing. This patch makes the comments more clearer.

[qcolombet]

LGTM

llvm/include/llvm/CodeGen/MachineFunction.h

public:
   // that the property hold, but not that it does not hold.
 
   // Property descriptions:
   // IsSSA: True when the machine function is in SSA form and virtual registers
   //  have a single def.
   // NoPHIs: The machine function does not contain any PHI instruction.
   // TracksLiveness: True when tracking register liveness accurately.
   //  While this property is set, register liveness information in basic block
-  //  live-in lists and machine instruction operands (e.g. kill flags, implicit
-  //  defs) is accurate. This means it can be used to change the code in ways
-  //  that affect the values in registers, for example by the register
-  //  scavenger.
-  //  When this property is clear, liveness is no longer reliable.
+  //  live-in lists and machine instruction operands (e.g. implicit defs) is
+  //  accurate, kill flags are conservatively accurate (kill flag correctly
+  //  indicates the last use of a register, an operand without kill flag may or
+  //  may not be the last use of a register). This means it can be used to
+  //  change the code in ways that affect the values in registers, for example
+  //  by the register scavenger.
+  //  When this property is cleared at a very late time, liveness is no longer
+  //  reliable.
   // NoVRegs: The machine function does not use any virtual registers.
   // Legalized: In GlobalISel: the MachineLegalizer ran and all pre-isel generic
   //  instructions have been legalized; i.e., all instructions are now one of:
   //   - generic and always legal (e.g., COPY)
   //   - target-specific
   //   - legal pre-isel generic instructions.
   // RegBankSelected: In GlobalISel: the RegBankSelect pass ran and all generic
   //  virtual registers have been assigned to a register bank.

llvm/include/llvm/CodeGen/MachineOperand.h

private:
   unsigned IsDef : 1;
 
   /// IsImp - True if this is an implicit def or use, false if it is explicit.
   /// This is only valid on register opderands.
   ///
   unsigned IsImp : 1;
 
   /// IsDeadOrKill
-  /// For uses: IsKill - True if this instruction is the last use of the
-  /// register on this path through the function.
+  /// For uses: IsKill - Conservatively indicates the last use of a register
+  /// on this path through the function. A register operand with true value of
+  /// this flag must be the last use of the register, a register operand with
+  /// false value may or may not be the last use of the register. After regalloc
+  /// we can use recomputeLivenessFlags to get precise kill flags.
   /// For defs: IsDead - True if this register is never used by a subsequent
   /// instruction.
   /// This is only valid on register operands.
   unsigned IsDeadOrKill : 1;
 
   /// See isRenamable().
   unsigned IsRenamable : 1;