DAGCombiner: Make the post-legalize vector op optimization more aggressive.

A legal BUILD_VECTOR goes in and gets constant folded into another legal
BUILD_VECTOR so we don't lose any legality here. The problematic PPC
optimization that made this check necessary was fixed recently.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175759 91177308-0d34-0410-b5e6-96231b3b80d8