Relax boolean processing in vect_maybe_update_slp_op_vectype

Relax boolean processing in vect_maybe_update_slp_op_vectype

The following makes VECTOR_BOOLEAN_TYPE_P processing consistent with
what we do without SLP.  The original motivation for rejecting of
VECTOR_BOOLEAN_TYPE_P extern defs was bad code generation.  But
the non-SLP codepath happily goes along - but always hits the
case of an uniform vector and this case specifically we can now
code-generate optimally.  So the following allows single-lane
externs as well.

Requiring patterns to code-generate can have bad influence on
the vectorization factor though a prototype patch of mine shows
that generating vector compares externally isn't always trivial.

The patch fixes the gcc.dg/vect/vect-early-break_82.c FAIL on x86_64
when --param vect-force-slp=1 is in effect.

	PR tree-optimization/117171
	* tree-vect-stmts.cc (vect_maybe_update_slp_op_vectype):
	Relax vect_external_def VECTOR_BOOLEAN_TYPE_P constraint.