aarch64: Fold vget_low_* intrinsics to BIT_FIELD_REF [PR102171]

aarch64: Fold vget_low_* intrinsics to BIT_FIELD_REF [PR102171]

This patch folds vget_low_* intrinsics to BIT_FILED_REF to open up more
optimization opportunities for gimple optimizers.

While we are here, we also remove the vget_low_* definitions from arm_neon.h and
use the new intrinsics framework.

	PR target/102171

gcc/ChangeLog:

	* config/aarch64/aarch64-builtins.cc (AARCH64_SIMD_VGET_LOW_BUILTINS):
	New macro to create definitions for all vget_low intrinsics.
	(VGET_LOW_BUILTIN): Likewise.
	(enum aarch64_builtins): Add vget_low function codes.
	(aarch64_general_fold_builtin): Fold vget_low calls.
	* config/aarch64/aarch64-simd-builtins.def: Delete vget_low builtins.
	* config/aarch64/aarch64-simd.md (aarch64_get_low<mode>): Delete.
	(aarch64_vget_lo_halfv8bf): Likewise.
	* config/aarch64/arm_neon.h (__attribute__): Delete.
	(vget_low_f16): Likewise.
	(vget_low_f32): Likewise.
	(vget_low_f64): Likewise.
	(vget_low_p8): Likewise.
	(vget_low_p16): Likewise.
	(vget_low_p64): Likewise.
	(vget_low_s8): Likewise.
	(vget_low_s16): Likewise.
	(vget_low_s32): Likewise.
	(vget_low_s64): Likewise.
	(vget_low_u8): Likewise.
	(vget_low_u16): Likewise.
	(vget_low_u32): Likewise.
	(vget_low_u64): Likewise.
	(vget_low_bf16): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/pr113573.c: Replace __builtin_aarch64_get_lowv8hi
	with vget_low_s16.
	* gcc.target/aarch64/vget_low_2.c: New test.
	* gcc.target/aarch64/vget_low_2_be.c: New test.

Signed-off-by: Pengxuan Zheng <quic_pzheng@quicinc.com>