[ARM] Better reductions

MVE has native reductions for integer add and min/max. The others need
to be expanded to a series of extract's and scalar operators to reduce
the vector into a single scalar. The default codegen for that expands
the reduction into a series of in-order operations.

This modifies that to something more suitable for MVE. The basic idea is
to use vector operations until there are 4 remaining items then switch
to pairwise operations. For example a v8f16 fadd reduction would become:
Y = VREV X
Z = ADD(X, Y)
z0 = Z[0] + Z[1]
z1 = Z[2] + Z[3]
return z0 + z1

The awkwardness (there is always some) comes in from something like a
v4f16, which is first legalized by adding identity values to the extra
lanes of the reduction, and which can then not be optimized away through
the vrev; fadd combo, the inserts remain. I've made sure they custom
lower so that we can produce the pairwise additions before the extra
values are added.

Differential Revision: https://reviews.llvm.org/D81397