[ARM,MVE] Use the new Tablegen `defvar` and `if` statements.

Summary:
This cleans up a lot of ugly `foreach` bodges that I've been using to
work around the lack of those two language features. Now they both
exist, I can make then all into something more legible!

In particular, in the common pattern in `ARMInstrMVE.td` where a
multiclass defines an `Instruction` instance plus one or more `Pat` that
select it, I've used a `defvar` to wrap `!cast<Instruction>(NAME)` so
that the patterns themselves become a little more legible.

Replacing a `foreach` with a `defvar` removes a level of block
structure, so several pieces of code have their indentation changed by
this patch. Best viewed with whitespace ignored.

NFC: the output of `llvm-tblgen -print-records` on the two affected
Tablegen sources is exactly identical before and after this change, so
there should be no effect at all on any of the other generated files.

Reviewers: MarkMurrayARM, miyuki

Reviewed By: MarkMurrayARM

Subscribers: kristof.beyls, hiraditya, dmgreen, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D72690