Fix RTL frontend handling of const_vectors

Fix RTL frontend handling of const_vectors

The RTL frontend makes sure that CONST_INTs use shared rtxes where
appropriate.  We should do the same thing for CONST_VECTORs,
reusing CONST0_RTX, CONST1_RTX and CONSTM1_RTX.  This also has
the effect of setting CONST_VECTOR_NELTS_PER_PATTERN and
CONST_VECTOR_NPATTERNS.

While looking at where to add that, I noticed we had some dead #includes
in read-rtl.c.  Some of the stuff that read-rtl-function.c does was once
in that file instead.

gcc/
	* read-rtl.c: Remove dead !GENERATOR_FILE block.
	* read-rtl-function.c (function_reader::consolidate_singletons):
	Generate canonical CONST_VECTORs.