bug#21901: bit shift wrong on maximal right shift

[Zefram]

With Guile 2.0.11:

scheme@(guile-user)> (ash 123 (ash -1 63))
$1 = 123

Correct result would of course be zero.  Problem only occurs for
exactly this shift distance: one bit less produces the right answer.
Problem also occurs on Guile 1.8.8.  Looking at the implementation,
the problem is attributable to the negation of the shift distance,
which in twos-complement fails to produce the expected positive result.

Note the resemblance to bug #14864, fixed in 2.0.10.  This bug is of
very similar form, but is distinct.  The test cases of #14864 pass for
me on the 2.0.11 that shows the problem with a 2^63 bit shift.  My bug
does occur with the rnrs bitwise-arithmetic-shift-right, which was used
in #14864, as well as with ash.

-zefram