flattenLet applied in bottomup traversal

Ensures deeply nested let-bindings are flattened first so that
let-expressions do not get inlined into argument positions in
their unflattened form.

Has some minor performance effects.

Before:
```
benchmarking normalization of examples/Reducer.hs
time                 395.2 ms   (385.3 ms .. 410.2 ms)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 401.3 ms   (396.8 ms .. 406.4 ms)
std dev              5.429 ms   (1.784 ms .. 7.434 ms)
variance introduced by outliers: 19% (moderately inflated)

benchmarking normalization of tests/shouldwork/Basic/AES.hs
time                 161.3 ms   (160.1 ms .. 162.5 ms)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 164.9 ms   (163.1 ms .. 169.4 ms)
std dev              4.042 ms   (490.2 μs .. 5.968 ms)
variance introduced by outliers: 12% (moderately inflated)
```

After:
```
benchmarking normalization of examples/Reducer.hs
time                 447.0 ms   (439.0 ms .. 451.0 ms)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 454.4 ms   (451.0 ms .. 460.9 ms)
std dev              6.411 ms   (13.17 μs .. 7.496 ms)
variance introduced by outliers: 19% (moderately inflated)

benchmarking normalization of tests/shouldwork/Basic/AES.hs
time                 182.2 ms   (180.8 ms .. 183.4 ms)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 185.3 ms   (183.9 ms .. 188.1 ms)
std dev              2.666 ms   (490.4 μs .. 3.715 ms)
variance introduced by outliers: 14% (moderately inflated)
```

Fixes #2598

changelog/2023-11-06T14_40_14+01_00_fix_2598

@@ -0,0 +1 @@
+FIXED: Name duplication in generated Verilog involving reset synchronizer [#2598](https://github.com/clash-lang/clash-compiler/issues/2598)

clash-lib/src/Clash/Normalize.hs

@@ -2,7 +2,7 @@
   Copyright   :  (C) 2012-2016, University of Twente,
                      2016     , Myrtle Software Ltd,
                      2017     , Google Inc.,
-                     2021-2022, QBayLogic B.V.
+                     2021-2023, QBayLogic B.V.
   License     :  BSD2 (see the file LICENSE)
   Maintainer  :  QBayLogic B.V. <[email protected]>
 
@@ -71,7 +71,8 @@ import           Clash.Normalize.Strategy
 import           Clash.Normalize.Transformations
 import           Clash.Normalize.Types
 import           Clash.Normalize.Util
-import           Clash.Rewrite.Combinators        ((>->),(!->),repeatR,topdownR)
+import           Clash.Rewrite.Combinators
+  ((>->), (!->), bottomupR, repeatR, topdownR)
 import           Clash.Rewrite.Types
   (RewriteEnv (..), RewriteState (..), bindings, debugOpts, extra,
    tcCache, topEntities, newInlineStrategy)
@@ -378,8 +379,8 @@ flattenCallTree (CBranch (nm,(Binding nm' sp inl pr tm r)) used) = do
                  apply "caseCon" caseCon >->
                  (apply "reduceConst" reduceConst !-> apply "deadcode" deadCode) >->
                  apply "reduceNonRepPrim" reduceNonRepPrim >->
-                 apply "removeUnusedExpr" removeUnusedExpr >->
-                 apply "flattenLet" flattenLet)) !->
+                 apply "removeUnusedExpr" removeUnusedExpr) >->
+               bottomupR (apply "flattenLet" flattenLet)) !->
       topdownSucR (apply "topLet" topLet)
 
     goCheap c@(CLeaf   (nm2,(Binding _ _ inl2 _ e _)))