X86: When combining shuffles just remove shuffles that are completely redundant.

CombineTo doesn't allow replacing a node with itself so this would crash if the
combined shuffle is the same as the input shuffle.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212181 91177308-0d34-0410-b5e6-96231b3b80d8