Alpha: Remove code duplication in block clear trailer

Remove code duplication in the part of `alpha_expand_block_clear' that
handles any aligned trailing part of the block, observing that the two
legs of code only differ by the machine mode and that we already take
the same approach with handling any unaligned prefix earlier on.  No
functional change, just code shuffling.

	gcc/
	* config/alpha/alpha.cc (alpha_expand_block_clear): Fold two
	legs of a conditional together.