[Ada] Fix small oversight in change to Optimize_Length_Comparison

[Pierre-Marie de Rodat]

The previous change has introduced a type consistency issue when the
length comparison boils down to a comparison of the upper bounds.

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

2020-06-17  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_ch4.adb (Optimize_Length_Comparison): Make sure the base
	types are the same when comparing Y'Last and X'Last directly.
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