[Ada] Fix handling of PATs

[Ada] Fix handling of PATs

[Pierre-Marie de Rodat]

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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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diff --git a/gcc/ada/exp_pakd.adb b/gcc/ada/exp_pakd.adb
--- a/gcc/ada/exp_pakd.adb
+++ b/gcc/ada/exp_pakd.adb
@@ -1909,9 +1909,10 @@ package body Exp_Pakd is
       --  where PAT is the packed array type. This works fine, since in the
       --  modular case we guarantee that the unused bits are always zeroes.
       --  We do have to compare the lengths because we could be comparing
-      --  two different subtypes of the same base type.
+      --  two different subtypes of the same base type. We can only do this
+      --  if the PATs on both sides are the same.
 
-      if Is_Modular_Integer_Type (PAT) then
+      if Is_Modular_Integer_Type (PAT) and then PAT = Etype (R) then
          Rewrite (N,
            Make_And_Then (Loc,
              Left_Opnd =>