[ARM,MVE] Add reversed isel patterns for MVE `vcmp qN,rN`

Summary:
As well as vector/vector compare instructions, MVE also has a family
of comparisons taking a vector and a scalar, which compare every lane
of the vector against the same value. We generate those at isel time
using isel patterns that match `(ARMvcmp vector, (ARMvdup scalar))`.

This commit adds corresponding patterns for the operand-reversed form
`(ARMvcmp (ARMvdup scalar), vector)`, with condition codes swapped as
necessary. That way, we can still generate the vector/scalar compare
instruction if the IR happens to have been rearranged to put the
operands the other way round, which can happen in some optimization
phases. Previously, a vcmp the other way round was handled by emitting
a `vdup` instruction to //explicitly// replicate the scalar input into
a vector, and then doing a vector/vector comparison.

I haven't added a new test, because it turned out that several
existing tests were already exhibiting that failure mode. So just
updating the expected output in the existing MVE codegen tests
demonstrates what's been improved.

Reviewers: ostannard, MarkMurrayARM, dmgreen

Reviewed By: dmgreen

Subscribers: kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70296

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