[SDAGBuilder] Make the AArch64 builder happier.

I intended this loop to only unwrap SplitVector actions, but it
was more broad than that, such as unwrapping WidenVector actions,
which makes operations seem legal when they're not.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237457 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed