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/reader/src/parser.rs

@@ -2279,10 +2279,6 @@ impl<'a> Parser<'a> {
                 let dst = self.match_regunit(ctx.unique_isa)?;
                 InstructionData::CopySpecial { opcode, src, dst }
             }
-            InstructionFormat::AdjustSpImm => {
-                let offset = self.optional_offset32()?;
-                InstructionData::AdjustSpImm { opcode, offset }
-            }
             InstructionFormat::RegSpill => {
                 let arg = self.match_value("expected SSA value operand")?;
                 self.match_token(