C++ PATCH for c++/65054 (ICE with pointer arithmetic)

[Jason Merrill]

In this testcase we were folding a bunch of nops within a 
POINTER_PLUS_EXPR down to a single nop, and then tripping the assert in 
maybe_constant_value that checks for useless rebuilding of the tree 
because cp_tree_equal ignores NOPs.  The reason for that choice seems to 
have been so that we can compare template non-type arguments both before 
and after conversion to match the type of the template parameter, so 
let's move that code into template_args_equal.

Tested x86_64-pc-linux-gnu, applying to trunk.
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