Another vec_merge simplification that's missing is transforming:
(vec_merge (vec_duplicate x) (vec_concat (y) (z)) (const_int N))
into
(vec_concat x z) if N == 1 (0b01) or
(vec_concat y x) if N == 2 (0b10)

For the testcase in this patch on aarch64 this allows us to try matching during combine the pattern:
(set (reg:V2DI 78 [ x ])
    (vec_concat:V2DI
        (mem:DI (reg/v/f:DI 76 [ y ]) [1 *y_4(D)+0 S8 A64])
        (mem:DI (plus:DI (reg/v/f:DI 76 [ y ])
                (const_int 8 [0x8])) [1 MEM[(long long int *)y_4(D) + 8B]+0 S8 A64])))

rather than the more complex:
(set (reg:V2DI 78 [ x ])
    (vec_merge:V2DI (vec_duplicate:V2DI (mem:DI (plus:DI (reg/v/f:DI 76 [ y ])
                    (const_int 8 [0x8])) [1 MEM[(long long int *)y_4(D) + 8B]+0 S8 A64]))
        (vec_duplicate:V2DI (mem:DI (reg/v/f:DI 76 [ y ]) [1 *y_4(D)+0 S8 A64]))
        (const_int 2 [0x2])))

We don't actually have an aarch64 pattern for the simplified version above, but it's a simple enough
form to add, so this patch adds such a pattern that performs a concatenated load of two 64-bit vectors
in adjacent memory locations as a single Q-register LDR. The new aarch64 pattern is needed to demonstrate
the effectiveness of the simplify-rtx change, so I've kept them together as one patch.

Now for the testcase in the patch we can generate:
construct_lanedi:
        ldr     q0, [x0]
        ret

construct_lanedf:
        ldr     q0, [x0]
        ret

instead of:
construct_lanedi:
        ld1r    {v0.2d}, [x0]
        ldr     x0, [x0, 8]
        ins     v0.d[1], x0
        ret

construct_lanedf:
        ld1r    {v0.2d}, [x0]
        ldr     d1, [x0, 8]
        ins     v0.d[1], v1.d[0]
        ret

The new memory constraint Utq is needed because we need to allow only the Q-register addressing modes but
the MEM expressions in the RTL pattern have 64-bit vector modes, and if we don't constrain them they will
allow the D-register addressing modes during register allocation/address mode selection, which will produce
invalid assembly.

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

	* simplify-rtx.c (simplify_ternary_operation, VEC_MERGE):
	Simplify vec_merge of vec_duplicate and vec_concat.
	* config/aarch64/constraints.md (Utq): New constraint.
	* config/aarch64/aarch64-simd.md (load_pair_lanes<mode>): New
	define_insn.

	* gcc.target/aarch64/load_v2vec_lanes_1.c: New test.

From-SVN: r254549