aarch64: Emit ADD X, Y, Y instead of SHL X, Y, #1 for Advanced SIMD

aarch64: Emit ADD X, Y, Y instead of SHL X, Y, #1 for Advanced SIMD

On many cores, including Neoverse V2 the throughput of vector ADD
instructions is higher than vector shifts like SHL.  We can lean on that
to emit code like:
  add     v0.4s, v0.4s, v0.4s
instead of:
  shl     v0.4s, v0.4s, 1

LLVM already does this trick.
In RTL the code gets canonincalised from (plus x x) to (ashift x 1) so I
opted to instead do this at the final assembly printing stage, similar
to how we emit CMLT instead of SSHR elsewhere in the backend.

I'd like to also do this for SVE shifts, but those will have to be
separate patches.

Signed-off-by: Kyrylo Tkachov <ktkachov@nvidia.com>
gcc/ChangeLog:

	* config/aarch64/aarch64-simd.md
	(aarch64_simd_imm_shl<mode><vczle><vczbe>): Rewrite to new
	syntax.  Add =w,w,vs1 alternative.
	* config/aarch64/constraints.md (vs1): New constraint.

gcc/testsuite/ChangeLog:

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