Cosmetic code contraction in evarsolve.ml + test sur unification avec let-in.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16344 85f007b7-540e-0410-9357-904b9bb8a0f7
Fixing unfolding of local definitions during unification that appeared
with commit r16233. This commit added a more precise filtering of
variables on which an evar was allowed to be dependent, but) but it
also broke some Ssreflect scripts. The reason why that filtering was
incorrectly applied, sometimes, to local definitions.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16346 85f007b7-540e-0410-9357-904b9bb8a0f7