[X86] Add helper function to reduce some code duplication when shrinking a...

[X86] Add helper function to reduce some code duplication when shrinking a vector load to a vzext_load.

There's more code for calling CombineTo and replacing the nodes
that I'd like to share, but its complicated by the getNode call
in the middle that needs to be specific to each opcode.

While there are also make sure we recursively delete the load
we're replacing. It eventually gets removed by a RemoveDeadNodes
call at the end of DAG combine, but we should be more eager about
it. We were inconsistently doing this in some places but not all.