This is an incremental improvement to the type narrowing in match.pd. 
It's largely based on the pattern I added to fix 47477.

Basically if we have

(bit_and (arith_op (convert A) (convert B)) mask)

Where the conversions are widening and the mask turns off all bits 
outside the original types of A & B, then we can turn that into

(bit_and (arith_op A B) mask)

We may need to convert A & B to an unsigned type with the same 
width/precision as their original type, but that's still better than a 
widening conversion.

Bootstrapped and regression tested on x86_64-linux-gnu.

OK for the trunk?