Fix EXTRACT_LAST_REDUCTION handling of pattern stmts

Fix EXTRACT_LAST_REDUCTION handling of pattern stmts

Unlike most vector ops, extract-last reductions replace the original
scalar code in-situ rather than adding an adjacent vector implementation.
I.e.:

   dest_1 = COND_EXPR <...>;

becomes:

   dest_1 = .EXTRACT_LAST (...);

gcc.dg/vect/vect-cond-reduc-4.c was ICEing for SVE because we tried
to replace the pattern statement in this way, rather than replacing
the original scalar statement.

2019-12-10  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-stmts.c (vect_finish_replace_stmt): Always use the
	original scalar statement rather than a pattern statement.
	(vectorizable_condition): Likewise, in the handling of extract-last
	reductions.

From-SVN: r279162