[ARM] Rename MVE instructions in Tablegen for consistency.

Summary:
Their names began with a mishmash of `MVE_`, `t2` and no prefix at
all. Now they all start with `MVE_`, which seems like a reasonable
choice on the grounds that (a) NEON is the thing they're most at risk
of being confused with, and (b) MVE implies Thumb-2, so a prefix
indicating MVE is strictly more specific than one indicating Thumb-2.

Reviewers: ostannard, SjoerdMeijer, dmgreen

Subscribers: javed.absar, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63492

llvm-svn: 363690