[committed] fix spurious strstr(s, "") warning

[James E Wilson <wilson at specifix dot com>]

The optimizer is converting
  cp = strstr (s, "");
to just
  cp = s;
Unfortunately, the argument s to strstr is const char *, and the result
of strstr is char *, so we have a type mismatch here after
optimization.  This generates a spurious warning when compiled with
-Wall.

The solution here is easy.  We just need to convert the argument to the
result type before returning it, so that effectively we are generating
  cp = (char *) s;

This is a regression from gcc-3.4.  This was broken between gcc-3.4 and
gcc-4.0, presumably as part of the tree-ssa work.  In gcc-3.4,
fold_builtin_strstr returns rtx.  In gcc-4.0, it returns a tree.  When
converted, we missed the fact that we needed to add a type conversion in
one place.  I scanned the rest of the code in builtins.c, but did not
find any other similar place that needed a fix.

This was tested with a mainline C and C++ bootstrap on an x86_64-linux
system, and gcc-4.0.x C and C++ bootstrap on an i386-freebsd4.10
system.  There were no regressions.
-- 
Jim Wilson, GNU Tools Support, http://www.specifix.com

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