Arm backend: Decompose sum in pass (#10852)

Moves the unrolling of reducing multiple indices
from the sum node visitor to a new DecomposeSumPass.

KeepDimsFalseToSqueezePass is merged into the new
pass to decompose the sum op fully in one pass.

This change introduces new rescales for each
reduced dim, requiring decomposition before quantization to get proper
quantization parameters.

Signed-off-by: Adrian Lundell <adrian.lundell@arm.com>