[llvm-mca] Correctly update the CyclesLeft of a register read in the presence...

[llvm-mca] Correctly update the CyclesLeft of a register read in the presence of partial register updates.

This patch fixe the logic in ReadState::cycleEvent(). That method was not
correctly updating field `TotalCycles`.

Added extra code comments in class ReadState to better describe each field.

llvm-svn: 334028