ext-dce: Fix subreg_lsb is_constant assumption (2)

This patch fixes another instance of the problem described in the
cover note for g:bf3037e923e9f91d93ab64bdf73a37f64f659fb9.

gcc/
	* ext-dce.cc (ext_dce_process_uses): Apply is_constant directly
	to the subreg_lsb.

(cherry picked from commit bddc41e290113dd9160b01f2fdf925a1876c8ee0)