[cfe] Replace use of IncrementalCompiler._userCode with local variables

This splits the usage of [_userCode] into `lastGoodKernelTarget` and
[currentKernelTarget], and renames the field to [_lastGoodKernelTarget].
This is done to make it clearer in what capacity the field is accessed.

Change-Id: Ie9344410ee62b649016a80e88bba731fa0a2344f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/221942
Commit-Queue: Johnni Winther <[email protected]>
Reviewed-by: Chloe Stefantsova <[email protected]>

Author: johnniwinther