Hexagon (decode): look for pkts with multiple insns at the same slot

Hexagon (decode): look for pkts with multiple insns at the same slot

Each slot in a packet can be assigned to at most one instruction.
Although the assembler generally ought to enforce this rule, we better
be safe than sorry and also do some check to properly throw an "invalid
packet" exception on wrong slot assignments.

This should also make it easier to debug possible future errors caused
by missing updates to `find_iclass_slots()` rules in
target/hexagon/iclass.c.

Co-authored-by: Taylor Simpson <tsimpson@quicinc.com>
Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Signed-off-by: Matheus Tavares Bernardino <quic_mathbern@quicinc.com>
Reviewed-by: Taylor Simpson <tsimpson@quicinc.com>
Tested-by: Taylor Simpson <tsimpson@quicinc.com>
Message-Id: <f8b829443523568823d062adf8bf6659bc6d4a3f.1683552984.git.quic_mathbern@quicinc.com>