aarch64: Use RTL builtins for [su]mlal_high intrinsics

aarch64: Use RTL builtins for [su]mlal_high intrinsics

Rewrite [su]mlal_high Neon intrinsics to use RTL builtins rather than
inline assembly code, allowing for better scheduling and
optimization.

gcc/ChangeLog:

2021-01-27  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64-simd-builtins.def: Add RTL builtin
	generator macros.
	* config/aarch64/aarch64-simd.md (*aarch64_<su>mlal_hi<mode>):
	Rename to...
	(aarch64_<su>mlal_hi<mode>_insn): This.
	(aarch64_<su>mlal_hi<mode>): Define.
	* config/aarch64/arm_neon.h (vmlal_high_s8): Use RTL builtin
	instead of inline asm.
	(vmlal_high_s16): Likewise.
	(vmlal_high_s32): Likewise.
	(vmlal_high_u8): Likewise.
	(vmlal_high_u16): Likewise.
	(vmlal_high_u32): Likewise.