Move emit and emit_safepoint to prelude.isle

Even though the implementation of emit and emit_safepoint may
be platform-specific, the interface ought to be common so that
other code in prelude.isle may safely call these constructors.

This patch moves the definition of emit (from all platforms)
and emit_safepoint (s390x only) to prelude.isle.  This required
adding an emit_safepoint implementation to aarch64 and x64 as
well - the latter is still a stub as special move mitosis
handling will be required.

cranelift/codegen/src/isa/s390x/inst.isle

@@ -1290,15 +1290,6 @@
 
 ;; Instruction creation helpers ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-;; Emit an instruction.
-;;
-;; This is low-level and side-effectful; it should only be used as an
-;; implementation detail by helpers that preserve the SSA facade themselves.
-(decl emit (MInst) Unit)
-(extern constructor emit emit)
-(decl emit_safepoint (MInst) Unit)
-(extern constructor emit_safepoint emit_safepoint)
-
 ;; Helper for emitting `MInst.AluRRR` instructions.
 (decl alu_rrr (Type ALUOp Reg Reg) Reg)
 (rule (alu_rrr ty op src1 src2)
@@ -2195,11 +2186,6 @@
 (rule (debugtrap_impl)
       (SideEffectNoResult.Inst (MInst.Debugtrap)))
 
-(decl safepoint (SideEffectNoResult) ValueRegs)
-(rule (safepoint (SideEffectNoResult.Inst inst))
-      (let ((_ Unit (emit_safepoint inst)))
-        (value_regs_invalid)))
-
 
 ;; Helpers for handling boolean conditions ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 

cranelift/codegen/src/isa/s390x/lower/isle/generated_code.manifest

@@ -1,4 +1,4 @@
 src/clif.isle 9ea75a6f790b5c03
-src/prelude.isle 51d2aef2566c1c96
-src/isa/s390x/inst.isle d7bfd05fb4d4a66d
+src/prelude.isle 2bfcafbef6b29358
+src/isa/s390x/inst.isle 1d525c87f7c77c26
 src/isa/s390x/lower.isle 57dcc39cbab2d1c6