I was working on polishing some of Kai's work to eliminate 
shorten_compare and stumbled on this tiny missed optimization.

Basically this change allows us to see something like this:

    n = (short unsigned int) mode_size[(unsigned int) mode] * 8 <= 64 ? 
(int) ((short unsigned int) mode_size[(unsigned int) mode] * 8) : 64;


Note the (int) cast on the true arm of the COND_EXPR.  We factor that 
out and adjust the constant (which is obviously free) into:

    n = (int)((short unsigned int) mode_size[(unsigned int) mode] * 8 <= 
64 ? ((short unsigned int) mode_size[(unsigned int) mode] * 8) : 64);


Seems subtle, but now the existing optimizers can recognize it as:

!   n = (int) MIN_EXPR <(short unsigned int) mode_size[(unsigned int) 
mode] * 8, 64>;


In another case I saw we had the same kind of transformation occur, but 
there was already a type conversation outside the MIN_EXPR.  So we ended 
up with

(T1) (T2) MIN_EXPR < ... >

And we were able to prove the conversion to T2 was redundant resulting 
in just (T) MIN_EXPR < ... >


You could legitimately ask why not apply this when both arguments are 
converted.  That leads to recursion via fold-const.c which wants to 
shove the conversion back into the arms in that case.  Removing that 
code results in testsuite regressions.  I felt it was time to cut that 
thread a bit as the real goal here is to remove the shorten_* bits in 
c-common, not convert all of fold-const.c to match.pd :-)

Bootstrapped and regression tested on x86_64-linux-gnu.  OK for the trunk?