[ARM,MVE] Fix confusing MC names for MVE VMINA/VMAXA insns.

Summary:
A recent commit accidentally defined names like `MVE_VMAXAs8` as
instances of the multiclass `MVE_VMINA`, and vice versa. This has no
effect on the test suite, because nothing directly refers to those
instruction names (the isel patterns are generated in Tablegen using
`!cast<Instruction>(NAME)` inside a lower-level multiclass). But it
means that `llvm-mc -show-inst` was listing VMAXA as VMINA, and it
would also affect any further draft code gen patches that use those
instruction ids.

Reviewers: MarkMurrayARM, dmgreen, miyuki, ostannard

Reviewed By: dmgreen

Subscribers: kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73034