vect: Don't apply masks to operations on invariants [PR108979]

The loop body in the testcase contains an operation on invariants.
SLP detects this and can hoist/schedule the operation outside of
the loop.  However, after the fix for PR96373, we would try to
apply a loop mask to this operation, even though the mask is
defined in the loop.

The patch does what Richi suggested in the PR: suppress the
masking for externs and constants.

gcc/
	PR tree-optimization/108979
	* tree-vect-stmts.cc (vectorizable_operation): Don't mask
	operations on invariants.

gcc/testsuite/
	PR tree-optimization/108979
	* gfortran.dg/vect/pr108979.f90: New test.