[ARM] Add MVE vector load/store instructions.


[ARM] Add MVE vector load/store instructions.

This adds the rest of the vector memory access instructions. It
includes contiguous loads/stores, with an ordinary addressing mode
such as [r0,#offset] (plus writeback variants); gather loads and
scatter stores with a scalar base address register and a vector of
offsets from it (written [r0,q1] or similar); and gather/scatters with
a vector of base addresses (written [q0,#offset], again with
writeback). Additionally, some of the loads can widen each loaded
value into a larger vector lane, and the corresponding stores narrow
them again.

To implement these, we also have to add the addressing modes they
need. Also, in AsmParser, the isMem query function now has
subqueries isGPRMem and isMVEMem, according to which kind of base
register is used by a given memory access operand.

I've also had to add an extra check in checkTargetMatchPredicate in
the AsmParser, without which our last-minute check of rGPR register
operands against SP and PC was failing an assertion because Tablegen
had inserted an immediate 0 in place of one of a pair of tied register
operands. (This matches the way the corresponding check for MCK_rGPR
in validateTargetOperandClass is guarded.) Apparently the MVE load
instructions were the first to have ever triggered this assertion, but
I think only because they were the first to have a combination of the
usual Arm pre/post writeback system and the rGPR class in particular.

Reviewers: dmgreen, samparker, SjoerdMeijer, t.p.northover

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62680