Reenable an assert.

Reenable an assert.

Back in December an assert was commented out in the code (when a big PR
was done).
The issue was that in one case of a tail call test there was a LcLVar with
type of TYP_STRUCT with a single eightbyte.
This should not have happened since in such cses the type of the LclVar is
normalized to the type of the first structs' eightbyte.
Enabling the assert has no effect on the test in question - it passes.
I suspect the issue was resolved sometime back (in January) when we
discovered there were some bad merges in the big PR wiping . As I recall, at least
one was related to a taken out tail call fix.

Fixes issue 3360.

Commit migrated from https://github.com/dotnet/coreclr/commit/4411f0c2d1d7ffb826840d702812a798a0e42c7e