Bug in builtin_floor optimization

[Dale Johannesen]

There is some clever code in convert_to_real that converts

double d;
(float)floor(d)

to

floorf((float)d)

(on targets where floor and floorf are considered builtins.)
This is wrong, because the (float)d conversion normally uses
round-to-nearest and can round up to the next integer.  For example:

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with -O2.
The transformation is also done for ceil, round, rint, trunc and 
nearbyint.
I'm not a math guru, but it looks like ceil, rint, trunc and nearbyint 
are
also unsafe for this transformation.  round may be salvageable.
Comments?  Should I preserve the buggy behavior with -ffast-math?