aarch64: Split vec_selects of bottom elements into simple move

aarch64: Split vec_selects of bottom elements into simple move

In certain intrinsics use cases GCC leaves SETs of a bottom-element vec
select lying around:
        (vec_select:DI (reg:V2DI 34 v2 [orig:128 __o ] [128])
            (parallel [
                    (const_int 0 [0])
                ])))

This can be treated as a simple move in aarch64 when done between SIMD
registers for all normal widths.
These go through the aarch64_get_lane pattern.
This patch adds a splitter there to simplify these extracts to a move
that can, perhaps, be optimised a way.
Another benefit is if the destination is memory we can use a simpler STR
instruction rather than ST1-lane.

gcc/

	* config/aarch64/aarch64-simd.md (aarch64_get_lane<mode>):
	Convert to define_insn_and_split.  Split into simple move when moving
	bottom element.

gcc/testsuite/

	* gcc.target/aarch64/vdup_lane_2.c: Scan for fmov rather than
	dup.