aarch64: Add pattern to match zero-extending scalar result of ADDLV

aarch64: Add pattern to match zero-extending scalar result of ADDLV

The vaddlv_u8 and vaddlv_u16 intrinsics produce a widened scalar result (uint16_t and uint32_t).
The ADDLV instructions themselves zero the rest of the V register, which gives us a free zero-extension
to 32 and 64 bits, similar to how it works on the GP reg side.
Because we don't model that zero-extension in the machine description this can cause GCC to move the
results of these instructions to the GP regs just to do a (superfluous) zero-extension.
This patch just adds a pattern to catch these cases. For the testcases we can now generate no zero-extends
or GP<->FP reg moves, whereas before we generated stuff like:
foo_8_32:
        uaddlv  h0, v0.8b
        umov    w1, v0.h[0] // FP<->GP move with zero-extension!
        str     w1, [x0]
        ret

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

gcc/ChangeLog:

	* config/aarch64/aarch64-simd.md
	(*aarch64_<su>addlv<VDQV_L:mode>_ze<GPI:mode>): New pattern.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/simd/addlv_zext.c: New test.