[ARM] Extra MVE select(binop) patterns

[ARM] Extra MVE select(binop) patterns

This is very similar to 243970d03cace2, but handling a slightly
different form of predicated operations. When starting with a pattern of
the form select(p, BinOp(x, y), x), Instcombine will often transform
this to BinOp(x, select(p, y, 0)), where 0 is the identity value of the
binop (0 for adds/subs, 1 for muls, -1 for ands etc). This adds the
patterns that transforms those back into predicated binary operations.

There is also a very minor adjustment to tablegen null_frag in here, to
allow it to also be recognized as a PatLeaf node, so that it can be used
in MVE_TwoOpPattern to easily exclude the cases where we do not need the
alternate transform.

Differential Revision: https://reviews.llvm.org/D84091