aarch64: Add QI -> HI zero-extension for LDAPR

aarch64: Add QI -> HI zero-extension for LDAPR

This patch is a straightforward extension of the zero-extending LDAPR
pattern to represent QI -> HI load-extends. This maps down to a LDAPRB-W
instruction.
This lets us remove a redundant zero-extend in the new test function.

Bootstrapped and tested on aarch64-none-linux-gnu.

gcc/ChangeLog:

	* config/aarch64/atomics.md
	(*aarch64_atomic_load<ALLX:mode>_rcpc_zext):
	Use SD_HSDI for destination mode iterator.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/ldapr-zext.c: Add test for u8 to u16
	extension.