MLBEDSW-3602: Output mismatch on some mobilenet_v1 int8 and int16

 - The failing tests contain operations with dynamic tensors which
are not supported and therefore they should be placed on the CPU.
However, a bug in the removal of RESHAPEs which contain a dynamic
shape prevented this happening.
 - This change adds a check to make sure that RESHAPE ops with a
dynamic shape tensor are not removed and instead are placed on the
CPU.

Signed-off-by: Tim Hall <tim.hall@arm.com>
Change-Id: I2d7481f7f80f99a0f01df100d956933777e6875a