[DAGCombiner] clean up visitEXTRACT_VECTOR_ELT

This isn't quite NFC, but I don't know how to expose
any outward diffs from these changes. Mostly, this
was confusing because it used 'VT' to refer to the
operand type rather the usual type of the input node.

There's also a large block at the end that is dedicated 
solely to matching loads, but that wasn't obvious. This
could probably be split up into separate functions to
make it easier to see. 

It's still not clear to me when we make certain transforms 
because the legality and constant conditions are 
intertwined in a way that might be improved.

llvm-svn: 349095