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/docs/ir.rst

@@ -360,13 +360,12 @@ instruction in the EBB.
 .. autoinst:: brff
 .. autoinst:: br_table
 
-.. inst:: JT = jump_table EBB0, EBB1, ..., EBBn
+.. inst:: JT = jump_table [EBB0, EBB1, ..., EBBn]
 
     Declare a jump table in the :term:`function preamble`.
 
     This declares a jump table for use by the :inst:`br_table` indirect branch
-    instruction. Entries in the table are either EBB names, or ``0`` which
-    indicates an absent entry.
+    instruction. Entries in the table are EBB names.
 
     The EBBs listed must belong to the current function, and they can't have
     any arguments.