Preserve lexical blocks

[Y-Nak]

Depends on #555

Retain block boundaries and local lifetimes during backend lowering.
Fixes flattened blocks that allowed locals to escape their scope or changed nested and compound assignment behavior.

[mbenke]

Sadly this (still) leads to incorrect Yul:

$ ./runsol.sh test/examples/cases/block-shadowing.solc
Processing: test/examples/cases/block-shadowing.solc
Compiling to hull...
Emitting hull for contract BlockShadowing
Writing to /home/ben/work/review/build/output1.hull
Generating Yul...
writing output to /home/ben/work/review/build/block-shadowing.yul
Compiling to bytecode...
Error: Variable name x already taken in this scope.
  --> build/block-shadowing.yul:25:9:
   |
25 |         let x
   |         ^^^^^

Error: solc compilation failed
ben@trawa:~/work/review$ cat /home/ben/work/review/build/block-shadowing.yul
object "BlockShadowingDeploy" {
  code {
    function usr$$$N126$36_36_83_55_48_36_36_36_81_81_51_55_36_78_57_36_49_48_53_95_49_49_48_95_49_49_56_95_49_49_49_95_49_48_55_95_57_55_95_57_56_95_49_48_56_95_49_48_49_95_54_36_49_48_53_95_49_49_48_95_49_49_56_95_49_49_49_95_49_48_55_95_49_48_49_95_49_36_52_53_36_84_51_57_36_78_49_48_36_49_49_54_95_57_53_95_49_48_53_95_49_49_48_95_49_48_53_95_49_49_54_95_57_53_95_53_50_95_53_55_95_53_50_95_48_36_ () {
      usr$init_()
      leave
    }
    function usr$_start () {
      mstore(64, memoryguard(128))
      if lt(codesize(), datasize("BlockShadowingDeploy")) {revert(0, 0)}
      if callvalue() {mstore(0, 3046674083)
                      revert(28, 4)}
      usr$$$N126$36_36_83_55_48_36_36_36_81_81_51_55_36_78_57_36_49_48_53_95_49_49_48_95_49_49_56_95_49_49_49_95_49_48_55_95_57_55_95_57_56_95_49_48_56_95_49_48_49_95_54_36_49_48_53_95_49_49_48_95_49_49_56_95_49_49_49_95_49_48_55_95_49_48_49_95_49_36_52_53_36_84_51_57_36_78_49_48_36_49_49_54_95_57_53_95_49_48_53_95_49_49_48_95_49_48_53_95_49_49_54_95_57_53_95_53_50_95_53_55_95_53_50_95_48_36_()
      let size := datasize("BlockShadowing")
      codecopy(0, dataoffset("BlockShadowing"), datasize("BlockShadowing"))
      return(0, size)
    }
    function usr$init_ () { }
    usr$_start()
  }
  object "BlockShadowing" {
    code {
      function usr$main () -> _result {
        let x
        x := 1
        let x
        x := 2
        let directShadow
        directShadow := 1
        let inlineShadow
        inlineShadow := 1
        let inlineUpdate
        inlineUpdate := 3
        let updateBeforeShadow
        updateBeforeShadow := 2
        x := 3
        _result := 3
        leave
      }
      let _mainresult := usr$main()
      mstore(0, _mainresult)
      return(0, 32)
    }
  }
}

[mbenke]

Proposed fix is on the yul-nesting-fix branch (commit 0b7fb9e)