driver: keep -O2 loops for the i64-less target

Scripts/tms-clang runs the generic opt pipeline without a target machine, so
the target cannot veto IndVarSimplify's exit-value replacement through TTI.
That rewrite turns sumto's closed form into an i33 multiply, which llc cannot
select.  Disable only that rewrite with -replexitval=never and document why.

scripts/tms-clang

@@ -49,7 +49,15 @@ trap 'rm -rf "$TMP"' EXIT HUP INT TERM
 "$TMS_BITADDR" --scale "$TMP/step1.ll" "$TMP/scaled.ll" 2>/dev/null
 
 if [ "$OPTIMIZE" -eq 1 ]; then
-    "$TMS_OPT" -passes='default<O2>' -S "$TMP/scaled.ll" -o "$TMP/opt.ll"
+    # This target has no i64 and no multiply-high, so IndVarSimplify's
+    # exit-value replacement (which rewrites a loop's final value into a
+    # closed-form i33 multiply, e.g. n*(n+1)/2 for sumto) produces IR that
+    # llc cannot select.  -replexitval is an option of IndVarSimplify; disabling
+    # only that rewrite keeps the loop, which is what ordinary -O2 code needs.
+    # This is a driver policy, not a target TTI policy: opt runs without a
+    # target machine here.
+    "$TMS_OPT" -passes='default<O2>' -replexitval=never -S "$TMP/scaled.ll" \
+        -o "$TMP/opt.ll"
     NORMALIZE_IN="$TMP/opt.ll"
 else
     NORMALIZE_IN="$TMP/scaled.ll"