[X86] Split combineGatherScatter into a version for generic ISD nodes and another...

[X86] Split combineGatherScatter into a version for generic ISD nodes and another version for X86 specific nodes.

The majority of the code doesn't run on the X86 nodes today since
its gated by isBeforeLegalizeOps and we don't formm X86 nodes
until after that. Except for a couple special case in type
legalization. But I think we would probably break those if
some of the transforms fire on them.

I want to remove the hardcoded operand numbers and the unusual
use of UpdateNodeOperands. Being able to know which ISD opcodes
are present should help with that.

llvm-svn: 373136