middle-end: ensure that VEC_PERM operands get lowered to the same SSA_NAME. [PR107717]

At the moment when the VEC_PERMs generated by this match.pd rule is generated
it creates two different SSA_NAMEs for the folded operand.  Because of this it
the permute switches from a single operand permute to a two operand permute and
the target may no longer support a permute for this.

This fixes it by ensuring we generate the same SSA_NAME for both operands.

gcc/ChangeLog:

	PR tree-optimization/107717
	* match.pd: Ensure same SSA_NAME.

gcc/testsuite/ChangeLog:

	PR tree-optimization/107717
	* gcc.target/aarch64/sve2/pr107717.c: New test.