[llvm-mca] Minor refactoring. NFCI

Also, removed a couple of unused methods from class Instruction.

llvm-svn: 328198
diff --git a/llvm/tools/llvm-mca/Instruction.cpp b/llvm/tools/llvm-mca/Instruction.cpp
index edf321a..de7c5c6 100644
--- a/llvm/tools/llvm-mca/Instruction.cpp
+++ b/llvm/tools/llvm-mca/Instruction.cpp
@@ -108,7 +108,7 @@
   Stage = IS_EXECUTING;
 
   // Set the cycles left before the write-back stage.
-  setCyclesLeft(Desc.MaxLatency);
+  CyclesLeft = Desc.MaxLatency;
 
   for (UniqueDef &Def : Defs)
     Def->onInstructionIssued();