[AArch64] Match FI+offset in STNP addressing mode.

[AArch64] Match FI+offset in STNP addressing mode.

First, we need to teach isFrameOffsetLegal about STNP.
It already knew about the STP/LDP variants, but those were probably
never exercised, because it's only the load/store optimizer that
generates STP/LDP, and the only user of the method is frame lowering,
which runs earlier.
The STP/LDP cases were wrong: they didn't take into account the fact
that they return two results, not one, so the immediate offset will be
the 4th operand, not the 3rd.

Follow-up to r247234.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247236 91177308-0d34-0410-b5e6-96231b3b80d8