In the testcase in this patch we create an SLP vector with only two
elements. Our current vector initialisation code will first duplicate
the first element to both lanes, then overwrite the top lane with a new
value.

This duplication can be clunky and wasteful.

Better would be to simply use the fact that we will always be
overwriting the remaining bits, and simply move the first element to the corrcet
place (implicitly zeroing all other bits).

This reduces the code generation for this case, and can allow more
efficient addressing modes, and other second order benefits for AArch64
code which has been vectorized to V2DI mode.

Note that the change is generic enough to catch the case for any vector
mode, but is expected to be most useful for 2x64-bit vectorization.

Unfortunately, on its own, this would cause failures in
gcc.target/aarch64/load_v2vec_lanes_1.c and
gcc.target/aarch64/store_v2vec_lanes.c , which expect to see many more
vec_merge and vec_duplicate for their simplifications to apply. To fix
this,
add a special case to the AArch64 code if we are loading from two memory
addresses, and use the load_pair_lanes patterns directly.

We also need a new pattern in simplify-rtx.c:simplify_ternary_operation
to catch:

  (vec_merge:OUTER
     (vec_duplicate:OUTER x:INNER)
     (subreg:OUTER y:INNER 0)
     (const_int N))

And simplify it to:

  (vec_concat:OUTER x:INNER y:INNER) or (vec_concat y x)

This is similar to the existing patterns which are tested in this
function, without requiring the second operand to also be a vec_duplicate. 

	* config/aarch64/aarch64.c (aarch64_expand_vector_init): Modify
	code generation for cases where splatting a value is not useful.
	* simplify-rtx.c (simplify_ternary_operation): Simplify
	vec_merge across a vec_duplicate and a paradoxical subreg forming a vector
	mode to a vec_concat.

	* gcc.target/aarch64/vect-slp-dup.c: New.


Co-Authored-By: Kyrylo Tkachov <kyrylo.tkachov@arm.com>

From-SVN: r260309