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