[X86] Stop using UpdateNodeOperands in combineGatherScatter. Create new nodes like...

[X86] Stop using UpdateNodeOperands in combineGatherScatter. Create new nodes like most other DAG combines.

Creating new nodes is what we usually do. Have to explicitly
check that we don't update to an existing node and having
to manually manage the worklist is unusual.

We can probably add a helper function to reduce the duplication
of having to check if we should create a gather or scatter, but
I wanted to just get the simple thing done.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@373137 91177308-0d34-0410-b5e6-96231b3b80d8