ARM64: refactor NEON post-indexed loads & stores (MC).

Previously, LLVM had no knowledge that these instructions actually
modified their address register: fine if they never end up in CodeGen,
but when I'd rather like to write some patterns for them it becomes a
disaster.

The change is mostly straightforward, I think the most significant
design decision was to *always* put the address write-back first. This
allows loads and stores to be accessed more uniformly, for example
permitting the continued sharing of the InstAlias definitions.

I also discovered that the custom Decode logic is no longer needed, so
I removed it.

No tests, because there should be no functionality change.

llvm-svn: 207839