RFA: PATCH to initializer_constant_valid_p for c++/38880

[Jason Merrill]

Fixing this bug in either fold or the front end had the same problem of 
making the pseudo-offsetof expression look like a valid C++ 
constant-expression, when it should not be.  We just want it to be 
accepted by initializer_constant_valid_p.  As it turns out, we just need 
to make the existing code accept PLUS_EXPR as well as POINTER_PLUS_EXPR, 
now that we no longer fold the former into the latter in this case.

Ian, was there any particular reason for limiting narrowing support to 
POINTER_PLUS_EXPR and MINUS_EXPR, but not PLUS_EXPR?  The testcase was 
failing because we fold the MINUS_EXPR to a PLUS_EXPR with the second 
operand negated.

Tested x86_64-pc-linux-gnu.

OK for trunk?
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