Rollup merge of #155630 - Zalathar:skip-filecheck, r=jieyouxu

Make `//@ skip-filecheck` a normal compiletest directive

The `skip-filecheck` directive is currently used by mir-opt tests, to suppress the default behaviour of running LLVM's `FileCheck` tool to check MIR output against FileCheck rules in the test file.

The `skip-filecheck` directive was not included in the big migration to `//@` directive syntax (rust-lang/rust#121370), perhaps because it was parsed and processed in the *miropt-test-tools* helper crate, not in compiletest itself.

Recently I noticed that a small number of *codegen-llvm* tests were using the `//@ build-pass` directive, which has the non-obvious effect of skipping FileCheck in codegen tests. That's quite confusing, so I decided to have the mir-opt tests migrate over to a proper `//@ skip-filecheck` directive, which could then be used by codegen tests as well.

(I also added skip-filecheck support to assembly tests, which are very similar to codegen tests, though there are currently no assembly tests that actually use `//@ skip-filecheck`.)

---

Support for using `//@ build-pass` in codegen tests to skip FileCheck was introduced in rust-lang/rust#113603. With hindsight, I think doing things that way was pretty clearly a  mistake, and we'll be better off with `//@ skip-filecheck`.

r? jieyouxu

Author: JonathanBrouwer

src/tests/directives.md

@@ -326,6 +326,8 @@ See [Pretty-printer](compiletest.md#pretty-printer-tests).
 The following directives affect how certain command-line tools are invoked, in
 test suites that use those tools:
 
+- `skip-filecheck` avoids running LLVM's `FileCheck` tool in tests that would normally run it to check output.
+  - Used by codegen tests, assembly tests, and mir-opt tests.
 - `filecheck-flags` adds extra flags when running LLVM's `FileCheck` tool.
   - Used by [codegen tests](compiletest.md#codegen-tests),
   [assembly tests](compiletest.md#assembly-tests), and