This simplifies the expression, especially for the rayon part, and also
lets us drop the `E: Copy` constraint.

`LLVMConstZext` recently got deleted, and it turns out (thanks to @nikic
for knowing!) that this is dead code. Tests all pass for me without this
logic, and per nikic:

> We always generate constants in "relocatable bag of bytes"
> representation, so you're never going to get a plain bool.

So this should be a safe thing to do.

r? @nikic
@rustbot label: +llvm-main

…avidtwco

Cleanup `rustc_mir_build/../check_match.rs`

The file had become pretty unwieldy, with a fair amount of duplication. As a bonus, I discovered that we weren't running some pattern checks in if-let chains.

I recommend looking commit-by-commit. The last commit is a whim, I think it makes more sense that way but I don't hold this opinion strongly.

Use `filter_map` in `try_par_for_each_in`

This simplifies the expression, especially for the rayon part, and also
lets us drop the `E: Copy` constraint.

…, r=nikic

consts: remove dead code around `i1` constant values

`LLVMConstZext` recently got deleted, and it turns out (thanks to `@nikic` for knowing!) that this is dead code. Tests all pass for me without this logic, and per nikic:

> We always generate constants in "relocatable bag of bytes"
> representation, so you're never going to get a plain bool.

So this should be a safe thing to do.

r? `@nikic`
`@rustbot` label: +llvm-main