Bug 78608 - gimple-ssa-sprintf.c:570:17: runtime error: negation
of -9223372036854775808 cannot be represented in type 'long int'
points out an integer overflow bug in the pass caught by ubsan.
The bug was due to negating a number without checking for equality
to INT_MIN.

In addition, my recent change to fix 78521 introduced a call to
abs() that broke the Solaris bootstrap:

   https://gcc.gnu.org/ml/gcc-patches/2016-12/msg00161.html

While fixing these two problems I noticed that the rest of the pass
wasn't handling the corner case of a width with the value of INT_MIN
specified via an argument to the asterisk, such as in:

   int n = snprintf(0, 0, "%*i", INT_MIN, 0);

This is undefined behavior because negative width is supposed to be
treated as the left justification flag followed by a positive width
(thus resulting in INT_MAX + 1 bytes).  This problem affected all
integer and floating point directives.

Finally, while there, I decided to include in information messages
a bit of detail about ranges of floating point values that was
missing.  I did this to help answer questions like those raised
earlier this week by Gerald here ("where does the 317 come from?):

   https://gcc.gnu.org/ml/gcc/2016-11/msg00102.html

The attached patch adjusts the pass to handle these problems.

Martin