rs6000: Fix an assertion in update_target_cost_per_stmt [PR103702]

rs6000: Fix an assertion in update_target_cost_per_stmt [PR103702]

This patch is to fix one wrong assertion which is too aggressive.
Vectorizer can do vec_construct costing for the vector type which
only has one unit.  For the failed case, the passed in vector type
is "vector(1) int", though it doesn't end up with any construction
eventually, we have to handle this kind of possibility.

gcc/ChangeLog:

	PR target/103702
	* config/rs6000/rs6000.cc
	(rs6000_cost_data::update_target_cost_per_stmt): Fix one wrong
	assertion with early return.

gcc/testsuite/ChangeLog:

	PR target/103702
	* gcc.target/powerpc/pr103702.c: New test.