[PowerPC] Avoid repeated hash lookups (NFC) (#131724)

Co-authored-by: Nikita Popov <[email protected]>

Author: kazutakahirata

llvm/lib/Target/PowerPC/PPCFrameLowering.cpp

@@ -2489,22 +2489,23 @@ bool PPCFrameLowering::spillCalleeSavedRegisters(
         if (Spilled[Dst])
           continue;
 
-        if (VSRContainingGPRs[Dst].second != 0) {
+        const auto &VSR = VSRContainingGPRs[Dst];
+        if (VSR.second != 0) {
           assert(Subtarget.hasP9Vector() &&
                  "mtvsrdd is unavailable on pre-P9 targets.");
 
           NumPESpillVSR += 2;
           BuildMI(MBB, MI, DL, TII.get(PPC::MTVSRDD), Dst)
-              .addReg(VSRContainingGPRs[Dst].first, getKillRegState(true))
-              .addReg(VSRContainingGPRs[Dst].second, getKillRegState(true));
-        } else if (VSRContainingGPRs[Dst].second == 0) {
+              .addReg(VSR.first, getKillRegState(true))
+              .addReg(VSR.second, getKillRegState(true));
+        } else if (VSR.second == 0) {
           assert(Subtarget.hasP8Vector() &&
                  "Can't move GPR to VSR on pre-P8 targets.");
 
           ++NumPESpillVSR;
           BuildMI(MBB, MI, DL, TII.get(PPC::MTVSRD),
                   TRI->getSubReg(Dst, PPC::sub_64))
-              .addReg(VSRContainingGPRs[Dst].first, getKillRegState(true));
+              .addReg(VSR.first, getKillRegState(true));
         } else {
           llvm_unreachable("More than two GPRs spilled to a VSR!");
         }