[X86] Allow X86ISD::Wrapper to be folded into the base of gather/scatter address

If the base of our gather corresponds to something contained in X86ISD::Wrapper we should be able to fold it into the address.

This patch refactors some of the address matching to more fully use the X86ISelAddressMode struct and the getAddressOperands helper. A new helper function matchVectorAddress is added to call matchWrapper or fall back to matchAddressBase.

We should also be able to support constant offsets from a wrapper, but I'll look into that in a future patch. We may even be able to completely reuse matchAddress here, but I wanted to start simple and work up to it.

Differential Revision: https://reviews.llvm.org/D39927

llvm-svn: 318057