ssa: Fix up maybe_rewrite_mem_ref_base complex type handling [PR116034]

The folding into REALPART_EXPR is correct, used only when the mem_offset
is zero, but for IMAGPART_EXPR it didn't check the exact offset value (just
that it is not 0).
The following patch fixes that by using IMAGPART_EXPR only if the offset
is right and using BITFIELD_REF or whatever else otherwise.

2024-07-23  Jakub Jelinek  <[email protected]>
	    Andrew Pinski  <[email protected]>

	PR tree-optimization/116034
	* tree-ssa.cc (maybe_rewrite_mem_ref_base): Only use IMAGPART_EXPR
	if MEM_REF offset is equal to element type size.

	* gcc.dg/pr116034.c: New test.

(cherry picked from commit b9cefd6)

Author: jakubjelinek

gcc/testsuite/gcc.dg/pr116034.c

@@ -0,0 +1,22 @@
+/* PR tree-optimization/116034 */
+/* { dg-do run } */
+/* { dg-options "-O1 -fno-strict-aliasing" } */
+
+int g;
+
+static inline int
+foo (_Complex unsigned short c)
+{
+  __builtin_memmove (&g, 1 + (char *) &c, 2);
+  return g;
+}
+
+int
+main ()
+{
+  if (__SIZEOF_SHORT__ == 2
+      && __CHAR_BIT__ == 8
+      && (foo (__BYTE_ORDER__ != __ORDER_BIG_ENDIAN__ ? 0x100 : 1)
+	  != (__BYTE_ORDER__ != __ORDER_BIG_ENDIAN__ ? 1 : 0x100)))
+    __builtin_abort ();
+}

gcc/tree-ssa.cc

@@ -1529,7 +1529,10 @@ maybe_rewrite_mem_ref_base (tree *tp, bitmap suitable_for_renaming)
 	}
       else if (TREE_CODE (TREE_TYPE (sym)) == COMPLEX_TYPE
 	       && useless_type_conversion_p (TREE_TYPE (*tp),
-					     TREE_TYPE (TREE_TYPE (sym))))
+					     TREE_TYPE (TREE_TYPE (sym)))
+	       && (integer_zerop (TREE_OPERAND (*tp, 1))
+		   || tree_int_cst_equal (TREE_OPERAND (*tp, 1),
+					  TYPE_SIZE_UNIT (TREE_TYPE (*tp)))))
 	{
 	  *tp = build1 (integer_zerop (TREE_OPERAND (*tp, 1))
 			? REALPART_EXPR : IMAGPART_EXPR,