JIT: Extra flags seen on switch node

[AndyAyersMS]

Failure seen in libraries testing with arm32/pgo (https://dev.azure.com/dnceng/public/_build/results?buildId=1163507&view=ms.vss-test-web.build-test-results-tab)

  Discovering: System.Formats.Cbor.Tests (method display = ClassAndMethod, method display options = None)
  Discovered:  System.Formats.Cbor.Tests (found 338 test cases)
  Starting:    System.Formats.Cbor.Tests (parallel test collections = on, max threads = 4)
Extra flags on tree [000021]: -CXG--------
               [000021] --CXG+-------             *  SWITCH    void  
               [000272] -------------             \--*  LCL_VAR   int    V20 tmp14        

Assert failure(PID 62 [0x0000003e], Thread: 68 [0x0044]): Assertion failed '!"Extra flags on tree"' in 'Microsoft.FSharp.Core.CompilerServices.GeneratedSequenceBase`1[__Canon][System.__Canon]:MoveNextImpl():bool:this' during 'Optimize layout' (IL size 121)

Likely fgOptimizeSwitchJumps needs to recompute the flags on the switch node.

[AndyAyersMS]

Can repro (w/ `COMPlus_TieredPGO=1):

    System.Formats.Cbor.Tests.CborPropertyTests.Roundtrip_IndefiniteTextString [STARTING]
Extra flags on tree [000002]: --XG--------
               [000002] ---XG+-------             *  SWITCH    void  
               [000021] -------------             \--*  LCL_VAR   int    V02 tmp1         

Assert failure(PID 43 [0x0000002b], Thread: 50 [0x0032]): Assertion failed '!"Extra flags on tree"' in 'loop@53-1[__Canon][System.__Canon]:get_CheckClose():bool:this' during 'Optimize layout' (IL size 52

we had

STMT00000 (IL 0x000...0x006)
               [000002] ---XG+-------             *  SWITCH    void  
               [000001] ---XG+-------             \--*  IND       int   
               [000010] -----+-------                \--*  ADD       byref 
               [000000] -----+-------                   +--*  LCL_VAR   ref    V00 this         
               [000009] -----+-------                   \--*  CNS_INT   int    24 field offset Fseq[pc]

and replaced the IND with a local as part of switch peeling:

Extra flags on tree [000002]: --XG--------
               [000002] ---XG+-------             *  SWITCH    void  
               [000021] -------------             \--*  LCL_VAR   int    V02 tmp1