Rearrange SLP nodes with duplicate statements [PR98138]

This change checks when a two_operators SLP node has multiple occurrences of
the same statement (e.g. {A, B, A, B, ...}) and tries to rearrange the operands
so that there are no duplicates. Two vec_perm expressions are then introduced
to recreate the original ordering. These duplicates can appear due to how
two_operators nodes are handled, and they prevent vectorization in some cases.

This targets the vectorization of the SPEC2017 x264 pixel_satd functions.
In some processors a larger than 10% improvement on x264 has been observed.

	PR tree-optimization/98138

gcc/ChangeLog:

	* tree-vect-slp.cc: Avoid duplicates in two_operators nodes.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/vect-slp-two-operator.c: New test.