The following testcase is miscompiled, because the postreload pass changes:
-(insn 14 13 23 2 (parallel [
-            (set (reg:DI 1 dx [94])
-                (plus:DI (reg:DI 1 dx [95])
-                    (reg:DI 5 di [92])))
-            (clobber (reg:CC 17 flags))
-        ]) "pr93402.c":8:30 186 {*adddi_1}
-     (expr_list:REG_EQUAL (plus:DI (reg:DI 5 di [92])
-            (const_int 111111111111 [0x19debd01c7]))
-        (nil)))
-(insn 23 14 25 2 (set (reg:SI 0 ax)
+(insn 23 13 25 2 (set (reg:SI 0 ax)
         (const_int 0 [0])) "pr93402.c":10:1 67 {*movsi_internal}
      (nil))
 (insn 25 23 26 2 (use (reg:SI 0 ax)) "pr93402.c":10:1 -1
      (nil))
-(insn 26 25 35 2 (use (reg:DI 1 dx)) "pr93402.c":10:1 -1
+(insn 26 25 35 2 (use (plus:DI (reg:DI 1 dx [95])
+            (reg:DI 5 di [92]))) "pr93402.c":10:1 -1
      (nil))
A USE insn is not a normal insn and verify_changes called from
apply_change_group is happy about any changes into it.
The following patch avoids this optimization if we were to change
the USE operand (this routine only changes a reg into (plus reg reg2)).

2020-01-23  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/93402
	* postreload.c (reload_combine_recognize_pattern): Don't try to adjust
	USE insns.

	* gcc.c-torture/execute/pr93402.c: New test.