[PowerPC] Fix VSX permutation isel

Not only did I invert the indices when I wrote the code, but I also did the
same thing when I wrote the regression test. Oops.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205046 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/PowerPC/vsx.ll

@@ -465,7 +465,7 @@ define <2 x double> @test54(<2 x double> %a, <2 x double> %b) {
   ret <2 x double> %v
 
 ; CHECK-LABEL: @test54
-; CHECK: xxpermdi 34, 34, 35, 1
+; CHECK: xxpermdi 34, 34, 35, 2
 ; CHECK: blr
 }