[SPIRV] Avoid repeated hash lookups (NFC) (#129358)

Author: kazutakahirata

llvm/lib/Target/SPIRV/SPIRVMergeRegionExitTargets.cpp

@@ -87,12 +87,8 @@ class SPIRVMergeRegionExitTargets : public FunctionPass {
       BasicBlock *RHSTarget =
           BI->isConditional() ? BI->getSuccessor(1) : nullptr;
 
-      Value *LHS = TargetToValue.count(LHSTarget) != 0
-                       ? TargetToValue.at(LHSTarget)
-                       : nullptr;
-      Value *RHS = TargetToValue.count(RHSTarget) != 0
-                       ? TargetToValue.at(RHSTarget)
-                       : nullptr;
+      Value *LHS = TargetToValue.lookup(LHSTarget);
+      Value *RHS = TargetToValue.lookup(RHSTarget);
 
       if (LHS == nullptr || RHS == nullptr)
         return LHS == nullptr ? RHS : LHS;