Fix up adjust_sp_imm instruction.

* Use imm64 rather than offset32
* Add predicate to enforce signed 32-bit limit to imm
* Remove AdjustSpImm format
* Add encoding tests for adjust_sp_imm
* Adjust use of adjust_sp_imm in Intel prologue_epilogue to match

lib/cretonne/src/verifier/mod.rs

@@ -359,7 +359,6 @@ impl<'a> Verifier<'a> {
             Store { .. } |
             RegMove { .. } |
             CopySpecial { .. } |
-            AdjustSpImm { .. } |
             Trap { .. } |
             CondTrap { .. } |
             NullAry { .. } => {}