[PowerPC] Avoid repeated hash lookups (NFC) (llvm#125634)

Author: kazutakahirata

llvm/lib/Target/PowerPC/PPCRegisterInfo.h

@@ -65,9 +65,10 @@ class PPCRegisterInfo : public PPCGenRegisterInfo {
   /// for a given imm form load/store opcode \p ImmFormOpcode.
   /// FIXME: move this to PPCInstrInfo class.
   unsigned getMappedIdxOpcForImmOpc(unsigned ImmOpcode) const {
-    if (!ImmToIdxMap.count(ImmOpcode))
+    auto It = ImmToIdxMap.find(ImmOpcode);
+    if (It == ImmToIdxMap.end())
       return PPC::INSTRUCTION_LIST_END;
-    return ImmToIdxMap.find(ImmOpcode)->second;
+    return It->second;
   }
 
   /// getPointerRegClass - Return the register class to use to hold pointers.