Use of REDUCE_BIT_FIELD in NOP_EXPR case

[Richard Kenner]

It looks wrong.  Consider converting sizetype to bitsizetype on a
64-bit machine.  We have TYPE_PRECISION (type) as 68.  Then we expand
the input, which will be DImode.  But calling_reduce_bit_field and
passing a 68-bit type with DImode will cause such oddities as shifts of
negative numbers.

I think the REDUCE_BIT_FIELD needs to be called after the conversion.
This came up compiling s-secsta.adb.