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/write.rs

@@ -407,7 +407,6 @@ pub fn write_operands(
                 write!(w, " %{} -> %{}", src, dst)
             }
         }
-        AdjustSpImm { offset, .. } => write!(w, " {}", offset),
         RegSpill { arg, src, dst, .. } => {
             if let Some(isa) = isa {
                 let regs = isa.register_info();