function unaryBlockSize( dtypeX, dtypeY ) {
	var nbx;
	var nby;

	nbx = bytesPerElement( dtypeX );
	nby = bytesPerElement( dtypeY );
	if ( nbx === null || nby === null ) { // e.g., "generic" arrays
		return defaults.BLOCK_SIZE_IN_ELEMENTS;
	}
	if ( nbx > nby ) {
		return ( defaults.BLOCK_SIZE_IN_BYTES/nbx )|0; // asm type annotation
	}
	return ( defaults.BLOCK_SIZE_IN_BYTES/nby )|0; // asm type annotation
}
Returns a loop block size for multi-dimensional array tiled loops.

@param {string} dtypeX - input array data type
@param {string} dtypeY - output array data type
@returns {integer} block size (in units of elements)

@example
var bsize = unaryBlockSize( 'float64', 'float64' );
// returns <number>