Add a tuple.drop text pseudoinstruction

[tlively]

We previously overloaded drop to mean both normal drops of single values and
also drops of tuple values. That works fine in the legacy text parser since it
can infer parent-child relationships directly from the s-expression structure of
the input, so it knows that a drop should drop an entire tuple if the
tuple-producing instruction is a child of the drop. The new text parser,
however, is much more like the binary parser in that it uses instruction types
to create parent-child instructions. The new parser always assumes that drop
is meant to drop just a single value because that's what it does in WebAssembly.

Since we want to continue to let Drop IR expressions consume tuples, and since
we will need a way to write tests for that IR pattern that work with the new
parser, introduce a new pseudoinstruction, tuple.drop, to represent drops of
tuples. This pseudoinstruction only exists in the text format and it parses to
normal Drop expressions. tuple.drop takes the arity of its operand as an
immediate, which will let the new parser parse it correctly in the future.

[tlively]

Current dependencies on/for this PR:

• main
  • PR Update tuple.make text format to include arity #6169
    • PR Add a tuple.drop text pseudoinstruction #6170 👈

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