[PR rtl-optimization/115877][2/n] Improve liveness computation for constant initializ...

[PR rtl-optimization/115877][2/n] Improve liveness computation for constant initialization

While debugging pr115877, I noticed we were failing to remove the destination
register from LIVENOW bitmap when it was set to a constant value.  ie  (set
(dest) (const_int)).  This was a trivial oversight in
safe_for_live_propagation.

I don't have an example of this affecting code generation, but it certainly
could.  More importantly, by making LIVENOW more accurate it's easier to debug
when LIVENOW differs from expectations.

As with the prior patch this has been tested as part of a larger patchset with
the crosses as well as individually on x86_64.

Pushing to the trunk,

	PR rtl-optimization/115877
gcc/
	* ext-dce.cc (safe_for_live_propagation): Handle RTX_CONST_OBJ.