aarch64: Remove redundant zero-extends with LDAR

aarch64: Remove redundant zero-extends with LDAR

Like other loads in AArch64, the LDARB,LDARH,LDAR instructions clear out the top part of their
destination register and we can thus avoid having to explicitly zero-extend it.
We were missing a combine pattern that this patch adds.

For one of the examples in the testcase we generated:
load_uint8_t_ext_uint16_t:
        adrp    x0, .LANCHOR0
        add     x0, x0, :lo12:.LANCHOR0
        ldarb   w0, [x0]
        and     w0, w0, 255
        ret

but now generate:
load_uint8_t_ext_uint16_t:
        adrp    x0, .LANCHOR0
        add     x0, x0, :lo12:.LANCHOR0
        ldarb   w0, [x0]
        ret

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

gcc/ChangeLog:

	* config/aarch64/atomics.md (*atomic_load<ALLX:mode>_zext<SD_HSDI:mode>):
	New pattern.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/ldar_2.c: New test.