Enable save/restore generation when there is a gap between registers
by inserting the missing ones at the moment when spill slots for
callee-saved registers are assigned.

Adjust stack pointer in two steps if offset is larger than 4096.
In the first step adjust stack pointer for the size necessary to
spill CSR onto the stack. In the second step adjust stack pointer
for the size necessary to spill local objects.

If function uses FP, it will now point to address -4096 from the
beginning of function's stack. After FP setup following offsets
will be relative to SP if function has no var-sized objects. If it
has var-sized objects offsets will be relative to FP.
Also, stack realignment now happens with INS instruction.