[NFC][msan] Extract handleSelectLikeInst (llvm#94881)

`blendv` instructions are very similar to `select`.
We will add support for them in followup patches.

llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp

@@ -4481,12 +4481,17 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
   }
 
   void visitSelectInst(SelectInst &I) {
-    IRBuilder<> IRB(&I);
     // a = select b, c, d
     Value *B = I.getCondition();
     Value *C = I.getTrueValue();
     Value *D = I.getFalseValue();
 
+    handleSelectLikeInst(I, B, C, D);
+  }
+
+  void handleSelectLikeInst(Instruction &I, Value *B, Value *C, Value *D) {
+    IRBuilder<> IRB(&I);
+
     Value *Sb = getShadow(B);
     Value *Sc = getShadow(C);
     Value *Sd = getShadow(D);