[gcc r13-3396] vect: Simplify first for shifted value generation [PR107240]

[Kewen Lin <linkw@gcc.gnu.org>]

https://gcc.gnu.org/g:5d6e834ecf212e7fc6f585b154af224d2e38c749

commit r13-3396-g5d6e834ecf212e7fc6f585b154af224d2e38c749
Author: Kewen Lin <linkw@linux.ibm.com>
Date:   Thu Oct 20 04:07:05 2022 -0500

    vect: Simplify first for shifted value generation [PR107240]
    
    As PR107240 shows, when both the value to be shifted and the
    count used for shifting are constants, it can be simplified
    to one constant value, and doesn't actually require to check
    if the current target supports vector shift operations or not.
    
    This patch uses a canonical way proposed by Richi to generate
    the shifted value, if it can be simplified, the shift operation
    would be gone, otherwise it's the same as before.
    
    It can help to fix the failures of vect-bitfield-write-{2,3}.c
    on Power.
    
            PR tree-optimization/107240
    
    2022-10-20  Richard Biener  <rguenther@suse.de>
    
    gcc/ChangeLog:
    
            * tree-vect-patterns.cc (vect_recog_bit_insert_pattern): Attempt to
            simplify shifted value first.

Diff:
---
 gcc/tree-vect-patterns.cc | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/gcc/tree-vect-patterns.cc b/gcc/tree-vect-patterns.cc
index 6afd57a50c4..d18e1e60996 100644
--- a/gcc/tree-vect-patterns.cc
+++ b/gcc/tree-vect-patterns.cc
@@ -2098,11 +2098,12 @@ vect_recog_bit_insert_pattern (vec_info *vinfo, stmt_vec_info stmt_info,
   tree shifted = value;
   if (shift_n)
     {
-      pattern_stmt
-	= gimple_build_assign (vect_recog_temp_ssa_var (container_type),
-			       LSHIFT_EXPR, value, shift);
-      append_pattern_def_seq (vinfo, stmt_info, pattern_stmt);
-      shifted = gimple_get_lhs (pattern_stmt);
+      gimple_seq stmts = NULL;
+      shifted
+	= gimple_build (&stmts, LSHIFT_EXPR, container_type, value, shift);
+      if (!gimple_seq_empty_p (stmts))
+	append_pattern_def_seq (vinfo, stmt_info,
+				gimple_seq_first_stmt (stmts));
     }
 
   tree mask_t