[arm] Fix BE index for single-var vector initialisers (PR91060)

[arm] Fix BE index for single-var vector initialisers (PR91060)

If a vector constructor has a single nonconstant element,
neon_expand_vector_init loads the constant lanes and then inserts the
nonconstant value.  The problem was that it was doing the insertion
using the arm_neon.h neon_vset_lane<mode> patterns, which use
architectural lane numbering rather than GCC lane numbering.

2019-07-10  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR target/91060
	* config/arm/iterators.md (V2DI_ONLY): New mode iterator.
	* config/arm/neon.md (vec_set<mode>_internal): Add a '@' prefix.
	(vec_setv2di_internal): Reexpress as...
	(@vec_set<V2DI_ONLY:mode>_internal): ...this.
	* config/arm/arm.c (neon_expand_vector_init): Use gen_vec_set_internal
	rather than gen_neon_vset_lane<mode>.

From-SVN: r273365