aarch64: Expand %<w> correctly according to mode iterator

aarch64: Expand %<w> correctly according to mode iterator

Pattern "*extend<SHORT:mode><GPI:mode>2_aarch64" is duplicated
from the corresponding zero_extend pattern, however %<w> needs
to be expanded according to its mode iterator because the smov
instruction is different to umov.

2021-08-09  Bin Cheng  <bin.cheng@linux.alibaba.com>

gcc/
	* config/aarch64/aarch64.md
	(*extend<SHORT:mode><GPI:mode>2_aarch64): Use %<GPI:w>0.