Add VIEW_CONVERT_EXPR to operand_equal_p

[Eric Botcazou]

> Added and comitted now.

Thanks.  Now on to the wrong code issues. :-)

Up to the change, the useless_type_conversion_p predicate was relying on 
structural equivalence via the TYPE_CANONICAL check, now it only looks at the 
outermost level (size, mode).  Now some back-ends, most notably x86-64, do a 
deep structural scan to determine the calling conventions (classify_argument) 
instead of just looking at the size and the mode, so consistency dictates that 
the type of the argument and that of the parameter be structurally equivalent 
and this sometimes can only be achieved by a VCE... which is now deleted. :-(
See the call to derivedIP in the attached testcase which now fails on x86-64.

How do we get away from here?


	* gnat.dg/discr44.adb: New test.

-- 
Eric Botcazou
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