[committed][PR rtl-optimization/115876] Fix one of two ubsan reported issues in new ext-dce.cc code

[Jeff Law]

David Binderman did a bootstrap build with ubsan enabled which triggered 
a few errors in the new ext-dce.cc code.  This fixes the trivial case of 
shifting negative values.

Bootstrapped and regression tested on x86.

Pushing to the trunk.

Jeff
        PR rtl-optimization/115876
        * ext-dce.cc (carry_backpropagate): Make mask and mmask unsigned.

diff --git a/gcc/ext-dce.cc b/gcc/ext-dce.cc
index adc9084df57..91789d283fc 100644
--- a/gcc/ext-dce.cc
+++ b/gcc/ext-dce.cc
@@ -374,13 +374,13 @@ binop_implies_op2_fully_live (rtx_code code)
    exclusively pertain to the first operand.  */
 
 HOST_WIDE_INT
-carry_backpropagate (HOST_WIDE_INT mask, enum rtx_code code, rtx x)
+carry_backpropagate (unsigned HOST_WIDE_INT mask, enum rtx_code code, rtx x)
 {
   if (mask == 0)
     return 0;
 
   enum machine_mode mode = GET_MODE_INNER (GET_MODE (x));
-  HOST_WIDE_INT mmask = GET_MODE_MASK (mode);
+  unsigned HOST_WIDE_INT mmask = GET_MODE_MASK (mode);
   switch (code)
     {
     case PLUS: