Extract node pair generation from tryAlloc

Currently, tryAlloc generates the list of allocation nodes or node
pairs (depending on how many nodes are requested) internally. This
patch moves the generation into a new function genAllocNodes, for both
code cleanup and future performance improvements.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>