cranelift: ISLE wrapper for constructing constants

[Gonzalosilvalde]

Description:

This pull request addresses issue #6038 by adding an ISLE wrapper for constructing constants. The goal is to allow mid-end optimization rules to generate constants generically without needing to know whether to use iconst, vconst, f32const, f64const, etc.

Changes:

• Added a helper function ty_vec in cranelift/codegen/src/isle_prelude.rs
• Defined an extractor ty_vec in `cranelift/codegen/src/prelude.isle
• Introduced a new rule in cranelift/codegen/src/prelude_opt.isle for constructing constant nodes

Additional Notes:

• It compiles without any problem and passes the cranelift test.

[cfallin]

@Gonzalosilvalde thanks for the PR!

In order to ensure this is tested and works for our lowerings, would you mind switching over at least a few existing uses of iconst/etc to use this instead? Looking at the issue(s) that spawned this might help to find cases to demonstrate it as well.