[PR 18672]: Fix gimplificication ICE

[Nathan Sidwell]

PR c++/18672 concerns some templatized c++ code.  However the fault
is not template based, and as you'll see I have a non-template test
case.

The bug is in canonicalize_addr_expr, which presumes an array's
elements cannot have incomplete type.  That is incorrect as
	struct X;
	extern struct X ary[];
is perfectly legal C and C++.  when the address of ARY is taken,
C produces an expression of type 'struct X *', whereas C++ produces
one of type 'struct X (*)[]', and that confuses the canonicalizer.

Whilst there I noticed some redundant checks in gimplify_conversion.
That function's comment says the EXPR is already a NOP_EXPR or
CONVERT_EXPR, and indeed its only caller enforces that.

booted & tested on i686-pc-linux-gnu, ok?

nathan
-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::     CodeSourcery LLC
nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk

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