s390x: Refactor branch and jumptable emission

The BranchTarget abstraction is no longer needed, since all branches are
being emitted using a MachLabel target.  Remove BranchTarget and simply
use MachLabel everywhere a branch target is required.  (This brings the
s390x back-end in line with what x64 does as well.)

In addition, simplify jumptable emission by moving all instructions
that do not depend on the internal label (i.e. the conditional branch
to the default label, as well as the scaling the index register) out of
the combined JTSequence instruction.

This refactoring will make moving branch generation to ISLE easier.

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

@@ -589,15 +589,15 @@
 
     ;; An unconditional branch.
     (Jump
-      (dest BranchTarget))
+      (dest MachLabel))
 
     ;; A conditional branch. Contains two targets; at emission time, both are emitted, but
     ;; the MachBuffer knows to truncate the trailing branch if fallthrough. We optimize the
     ;; choice of taken/not_taken (inverting the branch polarity as needed) based on the
     ;; fallthrough at the time of lowering.
     (CondBr
-      (taken BranchTarget)
-      (not_taken BranchTarget)
+      (taken MachLabel)
+      (not_taken MachLabel)
       (cond Cond))
 
     ;; A conditional trap execute a `Trap` if the condition is true. This is
@@ -624,7 +624,7 @@
     ;;
     ;; See, e.g., the lowering of `trapif` (conditional trap) for an example.
     (OneWayCondBr
-      (target BranchTarget)
+      (target MachLabel)
       (cond Cond))
 
     ;; An indirect branch through a register, augmented with set of all
@@ -644,10 +644,8 @@
     ;; Jump-table sequence, as one compound instruction (see note in lower.rs
     ;; for rationale).
     (JTSequence
-      (info BoxJTSequenceInfo)
       (ridx Reg)
-      (rtmp1 WritableReg)
-      (rtmp2 WritableReg))
+      (targets VecMachLabel))
 
     ;; Load an inline symbol reference with RelocDistance::Far.
     (LoadExtNameFar
@@ -680,8 +678,8 @@
 
 (type BoxCallInfo (primitive BoxCallInfo))
 (type BoxCallIndInfo (primitive BoxCallIndInfo))
+(type MachLabel (primitive MachLabel))
 (type VecMachLabel (primitive VecMachLabel))
-(type BranchTarget (primitive BranchTarget))
 (type BoxJTSequenceInfo (primitive BoxJTSequenceInfo))
 (type BoxExternalName (primitive BoxExternalName))
 (type ValueLabel (primitive ValueLabel))