[IRTranslator] Simplify fixed vector ConstantAggregateZero handling. NFC (#108667)

We don't need to loop through the elements, they're all the same zero.
We can get the first element and create a splat build_vector.

Author: topperc

llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp

@@ -3532,14 +3532,11 @@ bool IRTranslator::translate(const Constant &C, Register Reg) {
       return false;
     // Return the scalar if it is a <1 x Ty> vector.
     unsigned NumElts = CAZ->getElementCount().getFixedValue();
+    Constant &Elt = *CAZ->getElementValue(0u);
     if (NumElts == 1)
-      return translateCopy(C, *CAZ->getElementValue(0u), *EntryBuilder);
-    SmallVector<Register, 4> Ops;
-    for (unsigned I = 0; I < NumElts; ++I) {
-      Constant &Elt = *CAZ->getElementValue(I);
-      Ops.push_back(getOrCreateVReg(Elt));
-    }
-    EntryBuilder->buildBuildVector(Reg, Ops);
+      return translateCopy(C, Elt, *EntryBuilder);
+    // All elements are zero so we can just use the first one.
+    EntryBuilder->buildSplatBuildVector(Reg, getOrCreateVReg(Elt));
   } else if (auto CV = dyn_cast<ConstantDataVector>(&C)) {
     // Return the scalar if it is a <1 x Ty> vector.
     if (CV->getNumElements() == 1)