[RISCV] Add scalable vector icmp ISel patterns

Original patch by @rogfer01.

The RVV integer comparison instructions are defined in such a way that
many LLVM operations are defined by using the "opposite" comparison
instruction and swapping the operands. This is done in this patch in
most cases, except for the mappings where the immediate range must be
adjusted to accomodate:

    va < i --> vmsle{u}.vi vd, va, i-1, vm
    va >= i --> vmsgt{u}.vi vd, va, i-1, vm

That is left for future optimization; this patch supports all operations
but in the case of the missing mappings the immediate will be moved to
a scalar register first.

Since there are so many condition codes and operand cases to check, it
was decided to reduce the test burden by only testing the "vscale x 8"
vector types.

Authored-by: Roger Ferrer Ibanez <rofirrim@gmail.com>
Co-Authored-by: Fraser Cormack <fraser@codeplay.com>

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D94168

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