Enable fixed expression compilation tests, add more loop tests.

Tests are no longer failing since
#44235 is now fixed.

Change-Id: I2654233e4c62426e363eab518fdb5b041bc22724
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/173160
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Anna Gringauze <[email protected]>

Author: Anna Gringauze