[AArch64][SVE] NFCI: Choose consistent naming for predicated SDAG nodes

This patch proposes a naming convention for operations that take
a general predicate (and are thus predicated) that specifies
what happens to the false lanes.

Currently the _PRED suffix is used, which doesn't really say much other
than that it takes a predicate. In some instances this means it has
merging predication and in other cases it means zeroing-predication.

This patch also changes the order of operands to
AArch64ISD::DUP_MERGE_PASSTHRU, to pass the predicate as the first
operand, which is in line with all other predicates nodes. It takes the
passthru value as an explicit passthru value, which is always passed as
the last operand.

Reviewers: paulwalker-arm, cameron.mcinally, eli.friedman, dancgr, efriedma

Reviewed By: paulwalker-arm

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81850