Improve ABCREM on 64-bits architectures

Array indexes can be 32-bits integers even on 64-bits
architectures. This causes most loops to fail Array Bounds Checking
REMoval because the index is sign-extended before the check (see
mini_emit_ldelema_1_ins).

Sign-extending an I4 to I8 preserves its value, hence it is possible
to propagate the relation between the variables.
Merge pull request #852 from ranma42/abc-64bits

Improve ABCREM