ext-dce: fix off-by-one in subreg liveness for 32-bit modes

ext_dce_process_uses uses `size >= 32` to decide whether group 3
(bits 32-63) is live for a lowpart subreg source.  For SImode subregs
(size == 32), this incorrectly marks bits 32-63 as live, preventing
the pass from recognizing that the upper half of a DImode register is
dead.  This blocks lw -> lwu narrowing on RV64.

Change the condition to `size > 32`, consistent with the other
thresholds in the same block (size > 8, size > 16).  The size > 32
case is still reachable via SUBREG_PROMOTED_VAR_P which widens size
beyond the outer mode.

gcc/ChangeLog:

	* ext-dce.cc (ext_dce_process_uses): Fix off-by-one: use
	size > 32 instead of size >= 32 for group 3 liveness.

Co-authored-by: Konstantinos Eleftheriou <konstantinos.eleftheriou@vrull.eu>

Author: ptomsich

gcc/ext-dce.cc

@@ -1353,7 +1353,7 @@ ext_dce_process_uses (rtx_insn *insn, rtx obj,
 	    bitmap_set_bit (livenow, rn + 1);
 	  if (size > 16)
 	    bitmap_set_bit (livenow, rn + 2);
-	  if (size >= 32)
+	  if (size > 32)
 	    bitmap_set_bit (livenow, rn + 3);
 	  iter.skip_subrtxes ();
 	}