Use APInt::isAllOnesValue instead of popcnt. NFCI.

More obvious implementation and faster too.

llvm-svn: 284937
diff --git a/llvm/lib/Target/X86/X86ShuffleDecodeConstantPool.cpp b/llvm/lib/Target/X86/X86ShuffleDecodeConstantPool.cpp
index 2949816..1111552 100644
--- a/llvm/lib/Target/X86/X86ShuffleDecodeConstantPool.cpp
+++ b/llvm/lib/Target/X86/X86ShuffleDecodeConstantPool.cpp
@@ -82,7 +82,7 @@
 
     // Only treat the element as UNDEF if all bits are UNDEF, otherwise
     // treat it as zero.
-    if (EltUndef.countPopulation() == MaskEltSizeInBits) {
+    if (EltUndef.isAllOnesValue()) {
       UndefElts[i] = true;
       RawMask[i] = 0;
       continue;