s390x: Improved TrapIf implementation (#6079)

Following up on the discussion in
https://github.com/bytecodealliance/wasmtime/pull/6011
this adds an improved implementation of TrapIf for s390x
using a single conditional branch instruction.

If the trap conditions is true, we branch into the middle of
the branch instruction - those middle two bytes are zero,
which matches the encoding of the trap instruction.

In addition, show the trap code for Trap and TrapIf
instructions in assembler output.

cranelift/codegen/src/isa/s390x/inst/mod.rs

@@ -3185,11 +3185,13 @@ impl Inst {
                 let cond = cond.pretty_print_default();
                 format!("jg{} {}", cond, target)
             }
-            &Inst::Debugtrap => "debugtrap".to_string(),
-            &Inst::Trap { .. } => "trap".to_string(),
-            &Inst::TrapIf { cond, .. } => {
-                let cond = cond.invert().pretty_print_default();
-                format!("j{} 6 ; trap", cond)
+            &Inst::Debugtrap => ".word 0x0001 # debugtrap".to_string(),
+            &Inst::Trap { trap_code } => {
+                format!(".word 0x0000 # trap={}", trap_code)
+            }
+            &Inst::TrapIf { cond, trap_code } => {
+                let cond = cond.pretty_print_default();
+                format!("jg{} .+2 # trap={}", cond, trap_code)
             }
             &Inst::JTSequence { ridx, ref targets } => {
                 let ridx = pretty_print_reg(ridx, allocs);