Arm backend: Fix ensures check in UnsqueezeScalarPlaceholdersPass (#10811)

- In UnsqueezeScalarPlaceholdersPass, only the placeholders that meet
certain conditions will be unsqueezed. Otherwise, they retain their
original shape. This patch adds a new check to ensure that placeholders
that don't meet the conditions should be skipped

Signed-off-by: Yufeng Shi <yufeng.shi@arm.com>


cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

Signed-off-by: Yufeng Shi <yufeng.shi@arm.com>