Another GLIBC build error with GCC6

[Paul Eggert]

Roland McGrath wrote:
> the left shift doesn't even lose any bits, because the high
> bit is already zero (and it's a constant, so the compiler knows that).

But it does lose a bit.  It loses a zero bit, which can't be recovered the way 
that a non-overflowing signed left shift is recovered by shifting right the same 
amount.

> that's the whole point of the thing: to sign-extend the 31-bit value
> to 32 bits.  I don't think it should complain about 0x7fffffff << 1.

Such complaints are useful for compilers that take advantage of C's overflow 
rules to generate efficient-but-weird code when you violate the rules (something 
that describes GCC more and more nowadays...).

Anyway, if I understand this one correctly it should be easy enough to fix 
portably.  Something like the attached patch, say.  (I haven't tested it.)
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