aarch64: Fold builtins with highpart args to highpart equivalent [PR117850]

aarch64: Fold builtins with highpart args to highpart equivalent [PR117850]

Add a fold at gimple_fold_builtin to prefer the highpart variant of a builtin
if at least one argument is a vector highpart and all others are VECTOR_CSTs
that we can extend to 128-bits.

For example, we prefer to duplicate f0 and use UMULL2 here over DUP+UMULL:

uint16x8_t
foo (const uint8x16_t s)
{
    const uint8x8_t f0 = vdup_n_u8 (4);
    return vmull_u8 (vget_high_u8 (s), f0);
}

gcc/ChangeLog:
	PR target/117850
	* config/aarch64/aarch64-builtins.cc (LO_HI_PAIRINGS): New, group the
	lo/hi pairs from aarch64-builtin-pairs.def.
	(aarch64_get_highpart_builtin): New function.
	(aarch64_v128_highpart_ref): New function, helper to look for vector
	highparts.
	(aarch64_build_vector_cst): New function, helper to build duplicated
	VECTOR_CSTs.
	(aarch64_fold_lo_call_to_hi): New function.
	(aarch64_general_gimple_fold_builtin): Add cases for the lo builtins
	in aarch64-builtin-pairs.def.
	* config/aarch64/aarch64-builtin-pairs.def: New file, declare the
	parirs of lowpart-operating and highpart-operating builtins.

gcc/testsuite/ChangeLog:
	PR target/117850
	* gcc.target/aarch64/simd/vabal_combine.c: Removed.  This is
	covered by fold_to_highpart_1.c
	* gcc.target/aarch64/simd/fold_to_highpart_1.c: New test.
	* gcc.target/aarch64/simd/fold_to_highpart_2.c: Likewise.
	* gcc.target/aarch64/simd/fold_to_highpart_3.c: Likewise.
	* gcc.target/aarch64/simd/fold_to_highpart_4.c: Likewise.
	* gcc.target/aarch64/simd/fold_to_highpart_5.c: Likewise.
	* gcc.target/aarch64/simd/fold_to_highpart_6.c: Likewise.