AArch64: simplify illegal vector check. NFC.

Use a utility function to check whether the number of elements is a power of 2
and drop the redundant upper limit (a 128-bit vector with more than 16 elements
would have each element < 8 bits, not possible).

llvm-svn: 268422
diff --git a/clang/lib/CodeGen/TargetInfo.cpp b/clang/lib/CodeGen/TargetInfo.cpp
index db7c1f0..0777ba4 100644
--- a/clang/lib/CodeGen/TargetInfo.cpp
+++ b/clang/lib/CodeGen/TargetInfo.cpp
@@ -4614,7 +4614,7 @@
     unsigned NumElements = VT->getNumElements();
     uint64_t Size = getContext().getTypeSize(VT);
     // NumElements should be power of 2 between 1 and 16.
-    if ((NumElements & (NumElements - 1)) != 0 || NumElements > 16)
+    if (!llvm::isPowerOf2_32(NumElements))
       return true;
     return Size != 64 && (Size != 128 || NumElements == 1);
   }