[Ada] Improve optimization of "=" on bit-packed arrays

[Pierre-Marie de Rodat]

This patch fixes a performance regression, introduced by a previous bug
fix.  That fix had the unintended side effect of removing the
optimization in cases where the two operands are of two different
compiler-generated modular types.

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

gcc/ada/

	* exp_pakd.adb (Expand_Packed_Eq): Replace the check for *same*
	modular type, with a check for any modular type, and assert that
	the two types have the same modulus and so on.
	* exp_pakd.ads: Minor comment improvements.
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