rtl-optimization/113255 - avoid re-associating REG_POINTER MINUS

The following avoids re-associating

 (minus:DI (reg/f:DI 119)
    (minus:DI (reg/f:DI 120)
        (reg/f:DI 114)))

into

 (minus:DI (plus:DI (reg/f:DI 114)
        (reg/f:DI 119))
    (reg/f:DI 120))

as that possibly confuses the REG_POINTER heuristics of RTL
alias analysis.  This happens to miscompile the PRs testcase
during DSE which expands addresses via CSELIB which eventually
simplifies what it substituted to.  The original code does
the innocent ptr - (ptr2 - ptr2'), bias a pointer by the
difference of two other pointers.

gcc/simplify-rtx.cc

@@ -3233,11 +3233,15 @@ simplify_context::simplify_binary_operation_1 (rtx_code code,
          canonicalize (minus A (plus B C)) to (minus (minus A B) C).
 	 Don't use the associative law for floating point.
 	 The inaccuracy makes it nonassociative,
-	 and subtle programs can break if operations are associated.  */
+	 and subtle programs can break if operations are associated.
+	 Don't use the associative law when subtracting a MINUS from
+	 a REG_POINTER as that can trick find_base_term into discovering
+	 the wrong base.  */
 
       if (INTEGRAL_MODE_P (mode)
 	  && (plus_minus_operand_p (op0)
-	      || plus_minus_operand_p (op1))
+	      || ((!REG_P (op0) || !REG_POINTER (op0))
+		  && plus_minus_operand_p (op1)))
 	  && (tem = simplify_plus_minus (code, mode, op0, op1)) != 0)
 	return tem;