[Ada] Fix handling of PATs

[Pierre-Marie de Rodat]

When trying to compare two fields, one that's a packed array of known
length and one that's a packed array of unknown length, Expand_Packed_Eq
converts each side of the equality to its packed array type
and then says that's enough if the types are modular integer types.
That's correct, however, only if they're the same PATs.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* exp_pakd.adb (Expand_Packed_Eq): Fix handling of PATs.
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