[Bug rtl-optimization/115876] ext-dce.cc has ubsan issues; shifting negative values

[schwab--- via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115876

--- Comment #7 from Andreas Schwab <sch...@linux-m68k.org> ---
It broke the ASHIFT case.

-        return mask >> INTVAL (XEXP (x, 1));
+        return (HOST_WIDE_INT)mask >> INTVAL (XEXP (x, 1));