Z: reduce the number of registers used in VMnewEvaluator

VMnewEvaluator uses more registers than it needs.
Some registers like zeroReg are initialized but never used.
Some registers line dataSizeReg are redundant.
This commit uses existing registers as much as possible and
removes all the unused or redundant registers.
It also uses scratch register manager in genHeapAlloc for
better register allocation.

Signed-off-by: Ehsan Kiani Far <ehsan.kianifar@gmail.com>