[1/n] PR85694: Allow pattern definition statements to be reused

[1/n] PR85694: Allow pattern definition statements to be reused

This patch is the first part of a series to fix to PR85694.
Later patches can make the pattern for a statement S2 reuse the
results of a PATTERN_DEF_SEQ statement attached to an earlier
statement S1.  Although vect_mark_stmts_to_be_vectorized handled
this fine, vect_analyze_stmt and vect_transform_loop both skipped the
PATTERN_DEF_SEQ for S1 if S1's main pattern wasn't live or relevant.

I couldn't wrap my head around the flow in vect_transform_loop,
so ended up moving the per-statement handling into a subroutine.
That makes the patch look bigger than it actually is.

2018-06-20  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-stmts.c (vect_analyze_stmt): Move the handling of pattern
	definition statements before the early exit for statements that aren't
	live or relevant.
	* tree-vect-loop.c (vect_transform_loop_stmt): New function,
	split out from...
	(vect_transform_loop): ...here.  Process pattern definition
	statements without first checking whether the main pattern
	statement is live or relevant.

From-SVN: r261784