[x86] Inline an SSE4 helper function for INSERT_VECTOR_ELT lowering, no
functionality changed.

Separating this into two functions wasn't helping. There was a decent
amount of boilerplate duplicated, and some subsequent refactorings here
will pull even more common code out.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216644 91177308-0d34-0410-b5e6-96231b3b80d8