This commit adds the entire 8.1-M and MVE instruction set.
Most of the new instructions are MVE; those all live in a new tablegen
source file ARMInstrMVE.td. A few FP but non-MVE instructions are
added in ARMInstrVFP.td (VSCCLRM, and loads/stores/moves of a couple
of VFP system registers), and a few non-MVE and non-FP instructions in
ARMInstrThumb2.td (CLRM, the BF and LOL families, and CSEL).
The most complicated thing in the MVE instructions is that a lot of
them take a standard cluster of extra operations to support VPT
predication. Those are defined in ARMInstrFormats.td, under the names
'vpred_n' and 'vpred_r', with a comment explaining what they do.
A change to existing instructions in ARMInstrThumb2.td: tighten up bit
15 of the encodings of lots of normal data processing instructions
like shifted ORR. 8.1-M has reused some of the UNPREDICTABLE space in
those encodings, so now those bits have to be hard requirements.
As well as that, there's a load of supporting stuff in the C++,
supporting new formatting details (AsmPrinter), new register classes
and so on.