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.

mono/mini/abcremoval.c

@@ -275,6 +275,12 @@ get_relation_from_ins (MonoVariableRelationsEvaluationArea *area, MonoInst *ins,
 		value->value.variable.variable = ins->sreg1;
 		value->value.variable.delta = 0;
 		break;
+	case OP_SEXT_I4:
+		value->type = MONO_VARIABLE_SUMMARIZED_VALUE;
+		value->value.variable.variable = ins->sreg1;
+		value->value.variable.delta = 0;
+		value_kind = MONO_INTEGER_VALUE_SIZE_8;
+		break;
 	case OP_PHI:
 		value->type = MONO_PHI_SUMMARIZED_VALUE;
 		value->value.phi.number_of_alternatives = *(ins->inst_phi_args);