fortran: Pre-evaluate string pointers. [PR102043]

This avoids a regression on deferred_character_23.f90 later in the
patch series when array references are rewritten to use pointer
arithmetic.

The problem is a SAVE_EXPR tree as TYPE_SIZE_UNIT of one array element
type, which is used by the pointer arithmetic expressions.  As these
expressions appear in both branches of an if-then-else block, the tree
is lowered to a variable in one of the branches but it’s used in both
branches, which is invalid middle-end code.

This change pre-evaluates the array references or pointer arithmetics
to variables before the if-then-else block, so that the SAVE_EXPR are
expanded to variables in the parent scope of the if-then-else block,
and expressions referencing the variables remain valid in both
branches.

	PR fortran/102043

gcc/fortran/ChangeLog:
	* trans-expr.cc: Pre-evaluate src and dest to variables
	before using them.

gcc/testsuite/ChangeLog:
	* gfortran.dg/dependency_49.f90: Update variable occurence
	count.

gcc/fortran/trans-expr.cc

@@ -8093,6 +8093,13 @@ gfc_trans_string_copy (stmtblock_t * block, tree dlength, tree dest,
   cond2 = fold_build2_loc (input_location, LT_EXPR, logical_type_node, slen,
 			   dlen);
 
+  /* Pre-evaluate pointers unless one of the IF arms will be optimized away.  */
+  if (!CONSTANT_CLASS_P (cond2))
+    {
+      dest = gfc_evaluate_now (dest, block);
+      src = gfc_evaluate_now (src, block);
+    }
+
   /* Copy and pad with spaces.  */
   tmp3 = build_call_expr_loc (input_location,
 			      builtin_decl_explicit (BUILT_IN_MEMMOVE),

gcc/testsuite/gfortran.dg/dependency_49.f90

@@ -11,4 +11,5 @@ program main
   a%x = a%x(2:3)
   print *,a%x
 end program main
-! { dg-final { scan-tree-dump-times "__var_1" 4 "original" } }
+! The temporary var appears three times: declaration, copy-in and copy-out
+! { dg-final { scan-tree-dump-times "__var_1" 3 "original" } }