Fix internal error with Shift_Right operator on signed type

This is a regression present on the mainline and 10 branch in the form
of an ICE with a shift operator applied to a variable of a signed type,
and which is caused by a type mismatch.

gcc/ada/ChangeLog:
	* gcc-interface/trans.c (gnat_to_gnu) <N_Op_Shift>: Also convert
	GNU_MAX_SHIFT if the type of the operation has been changed.
	* gcc-interface/utils.c (can_materialize_object_renaming_p): Add
	pair of missing parentheses.

gcc/testsuite/ChangeLog:
	* gnat.dg/shift1.adb: New test.