[CodeGen] Avoid repeated hash lookups (NFC) (llvm#125463)

Author: kazutakahirata

llvm/include/llvm/CodeGen/LiveIntervals.h

@@ -149,8 +149,9 @@ class LiveIntervals {
   LiveInterval &createEmptyInterval(Register Reg) {
     assert(!hasInterval(Reg) && "Interval already exists!");
     VirtRegIntervals.grow(Reg.id());
-    VirtRegIntervals[Reg.id()] = createInterval(Reg);
-    return *VirtRegIntervals[Reg.id()];
+    auto &Interval = VirtRegIntervals[Reg.id()];
+    Interval = createInterval(Reg);
+    return *Interval;
   }
 
   LiveInterval &createAndComputeVirtRegInterval(Register Reg) {
@@ -168,8 +169,9 @@ class LiveIntervals {
 
   /// Interval removal.
   void removeInterval(Register Reg) {
-    delete VirtRegIntervals[Reg];
-    VirtRegIntervals[Reg] = nullptr;
+    auto &Interval = VirtRegIntervals[Reg];
+    delete Interval;
+    Interval = nullptr;
   }
 
   /// Given a register and an instruction, adds a live segment from that