[AArch64][GlobalISel] Add legalization for G_VECREDUCE_SEQ_FADD. (#76238)

And G_VECREDUCE_SEQ_FMUL at the same time. They require the elements of
the vector operand to be accumulated in order, so just need to be
scalarized.

Some of the operands are not simplified as much as they can quite yet
due to not canonicalizing constant operands post-legalization.