Before this, Cranelift ABI code would emit a stack-load instruction for
every stack argument and add all register arguments to the `args`
pseudo-instruction, whether those arguments were used or not.

However, we already know which arguments are used at that point because
we need the analysis for load-sinking, so it's easy to filter the unused
arguments out.

This avoids generating loads that are immediately dead, which is good
for the generated code. It also slightly reduces the size of the
register allocation problem, which is a small win in compile time.

This also changes which registers RA2 chooses in some cases because it
no longer considers unused defs from the `args` pseudo-instruction.

There was an existing method named `arg_is_needed_in_body` which sounded
like it should be the right place to implement this. However, that
method was only used for Baldrdash integration and has been a stub since
that integration was removed in bytecodealliance#4571. Also it didn't have access to the
`value_ir_uses` map needed here. But the place where that method was
called does have access to that map and was perfect for this.

Also, don't emit a `dummy_use` pseudo-instruction for the vmctx if it's
otherwise unused everywhere, as we want to drop it from the `args`
instruction in that case and then RA2 complains that it's used without
being defined.

Furthermore, don't emit debug info specially for the vmctx parameter,
because it's already emitted for all block parameters including vmctx.

Thanks to @elliottt for doing the initial investigation of this change
with me, and to @cfallin for helping me track down the `dummy_use` false
dependency.