[committed] Avoid right shifting signed value on ext-dce.cc

This is analogous to a prior patch to ext-dce which fixes propagation of sign
bits, but this time for the saturating variants.  I'd held off fixing those
because I wanted the time to look at that code (since we don't have a testcase
for it as far as I know).

Not surprisingly, putting an abort on that path and running an x86 bootstrap
and testsuite run, it never triggers.  Of course not a lot of code tries to do
saturating shifts.

Anyway, bootstrapped and regression tested on x86_64.  Pushing to the trunk.

Thanks for everyone's patience.

gcc/
	* ext-dce.cc (carry_backpropagate): Cast mask to HOST_WIDE_INT before
	shifting.

gcc/ext-dce.cc

@@ -556,7 +556,7 @@ carry_backpropagate (unsigned HOST_WIDE_INT mask, enum rtx_code code, rtx x)
 			     >> (INTVAL (XEXP (x, 1))
 				 + (XEXP (x, 1) != const0_rtx
 				    && code == SS_ASHIFT))))
-		  | (mask >> INTVAL (XEXP (x, 1))));
+		  | ((HOST_WIDE_INT)mask >> INTVAL (XEXP (x, 1))));
 	}
       return mmask;