Remove the concept of non-dense jump tables.

WebAssembly doesn't have non-dense jump tables, and higher-level users
are better served by the facilities in lib/frontend/src/switch.rs for
working with non-dense switches.

This eliminates the concept of "absent" jump table entries, which
were represented as "0" in the text format.

Also, jump table contents are now enclosed in `[` and `]`, so that
we can unambiguously display empty jump tables. Previously, empty jump
tables were displayed as if they had a single absent entry.

cranelift/filetests/isa/x86/binary64.clif

@@ -1405,7 +1405,7 @@ ebb0:
 
 ; Tests for i64 jump table instructions.
 function %I64_JT(i64 [%rdi]) {
-    jt0 = jump_table ebb1, ebb2, ebb3
+    jt0 = jump_table [ebb1, ebb2, ebb3]
 
 ebb0(v0: i64 [%rdi]):
     ; Note: The next two lines will need to change whenever instructions are

cranelift/filetests/isa/x86/legalize-br-table.clif

@@ -4,7 +4,7 @@ target x86_64
 
 function u0:0(i64) system_v {
     ss0 = explicit_slot 1
-    jt0 = jump_table ebb1
+    jt0 = jump_table [ebb1]
 
 ebb0(v0: i64):
     v1 = stack_addr.i64 ss0

cranelift/filetests/parser/branch.clif

@@ -81,8 +81,8 @@ ebb1(v92: i32, v93: f32):
 ; nextln: }
 
 function %jumptable(i32) {
-    jt200 = jump_table 0, 0
-    jt2 = jump_table 0, 0, ebb10, ebb40, ebb20, ebb30
+    jt200 = jump_table []
+    jt2 = jump_table [ebb10, ebb40, ebb20, ebb30]
 
 ebb10(v3: i32):
     br_table v3, ebb50, jt2
@@ -97,8 +97,8 @@ ebb50:
     trap user1    
 }
 ; sameln: function %jumptable(i32) fast {
-; check:      jt2 = jump_table 0, 0, ebb10, ebb40, ebb20, ebb30
-; check:      jt200 = jump_table 0
+; check:      jt2 = jump_table [ebb10, ebb40, ebb20, ebb30]
+; check:      jt200 = jump_table []
 ; check:  ebb10(v3: i32):
 ; nextln:     br_table v3, ebb50, jt2
 ; nextln: 

cranelift/filetests/verifier/type_check.clif

@@ -68,7 +68,7 @@ function %fn_call_incorrect_arg_type(i64) {
 ; TODO: Should we instead just verify that jump tables contain no EBBs that take arguments? This
 ; error doesn't occur if no instruction uses the jump table.
 function %jump_table_args() {
-    jt1 = jump_table ebb1
+    jt1 = jump_table [ebb1]
     ebb0:
         v0 = iconst.i32 0
         br_table v0, ebb2, jt1 ; error: takes no arguments, but had target ebb1 with 1 arguments

cranelift/filetests/wasm/control.clif

@@ -48,7 +48,7 @@ ebb0:
 }
 
 function %br_table(i32) {
-jt0 = jump_table ebb3, ebb1, 0, ebb2
+jt0 = jump_table [ebb3, ebb1, ebb2]
 
 ebb0(v0: i32):
     br_table v0, ebb4, jt0