middle-end: Fix PR103007, add missing check on complex fms detection.

middle-end: Fix PR103007, add missing check on complex fms detection.

The complex FMS detection is missing a check on if the nodes of the VEC_PERM
has the amount of children we expect before it recurses.

This check is there on MUL and FMA but was missing for FMS, due to this the
compiler goes on further than it should and hits an assert.

gcc/ChangeLog:

	PR tree-optimization/103007
	* tree-vect-slp-patterns.c (complex_fms_pattern::matches): Add elem
	check.

gcc/testsuite/ChangeLog:

	PR tree-optimization/103007
	* g++.dg/pr103007.C: New test.