[AArch64] Fix incorrect big-endian spill in foldMemoryOperandImpl (#65601)

When an sreg sub-register of a q register was spilled,
AArch64InstrInfo::foldMemoryOperandImpl would emit a spill of a d
register, which gives the wrong result when the target is big-endian as
the following q register fill will put the value in the top half.

Fix this by greatly simplifying the existing code for widening the spill
to only handle wzr to xzr widening, as the default result we get if the
function returns nullptr is already that a widened spill will be
emitted.