[patch] builtins.c: Fix PR tree-optimization/15738.

[Kazu Hirata]

Hi Jeff,

> Shouldn't we try to put all the simplification/optimization stuff
> into fold_builtin_strchr, then have expand_builtin_strchr and
> expand_builtin_strrchr call the folder to simplify/optimize the call?
> That would avoid a lot of duplication.
> 
> [ And yes, that applies to most of the fold_builtin_XXX stuff I
>   added a while back.  Let's start with fixing the simple ones ;-) ]

Yes, the code duplication is definitely a concern.  I am thinking
about moving as much expand-time builtins optimizations as possible to
tree level, removing those expand-time optimizations, and then having
fold_builtin_BLAH call simplify_builtin_BLAH (or vive versa).

Kazu Hirata