Test forward references in IntrinsicEmitter on Neon LD(2|3|4)

This patch tests the forward-referencing added in D62995 by changing
some existing intrinsics to use forward referencing of overloadable
parameters, rather than backward referencing.

This patch changes the TableGen definition/implementation of
llvm.aarch64.neon.ld2lane and llvm.aarch64.neon.ld2lane intrinsics
(and similar for ld3 and ld4). This change is intended to be
non-functional, since the behaviour of the intrinsics is
expected to be the same.

Reviewers: arsenm, dmgreen, RKSimon, greened, rnk

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D63189

llvm-svn: 363546