Add -fno-cse to T15426 and T18964

This -fno-cse change is to avoid these performance tests depending on
flukey CSE stuff.  Each contains several independent tests, and we don't
want them to interact.

See #23925.

By killing CSE we expect a 400% increase in T15426, and 100% in T18964.

Metric Increase:
    T15426
    T18964

testsuite/tests/perf/should_run/T15426.hs

+{-# OPTIONS_GHC -fno-cse #-}
+    -- Avoid depending on flukey CSE; there are really 5 independent
+    -- tests in this module, and we don't want them to interact.
+    -- See #23925
+
 import Control.Exception (evaluate)
 import qualified Data.List as L
 
@@ -28,4 +33,4 @@ As a result these lists are now floated out and shared.
 
 Just leaving breadcrumbs, in case we later see big perf changes on
 this (slightly fragile) benchmark.
--}
\ No newline at end of file
+-}

testsuite/tests/perf/should_run/T18964.hs

+{-# OPTIONS_GHC -fno-cse #-}
+    -- Avoid depending on flukey CSE; there are really 4 independent
+    -- tests in this module, and we don't want them to interact.
+    -- See #23925
+
 import GHC.Exts
 import Data.Int