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 (https://github.com/rust-lang/rust/pull/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`.)

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Support for using `//@ build-pass` in codegen tests to skip FileCheck was introduced in https://github.com/rust-lang/rust/pull/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