X86: Add a note.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175408 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/README-SSE.txt

@@ -953,3 +953,12 @@ similarly, v[0]-v[1] should match to hsubpd, and {v[0]-v[1], w[0]-w[1]} should
 turn into hsubpd also.
 
 //===---------------------------------------------------------------------===//
+
+define <2 x i32> @foo(<2 x double> %in) {
+  %x = fptosi <2 x double> %in to <2 x i32>
+  ret <2 x i32> %x
+}
+
+Should compile into cvttpd2dq instead of being scalarized into 2 cvttsd2si.
+
+//===---------------------------------------------------------------------===//