Rollup merge of #122257 - Nadrieril:tweak-mir-opt-tests, r=clubby789

mir-opt tests: don't run a different set on --bless; enable --keep-stage-std

Currently, `./x.py test tests/mir-opt` runs only the tests for the current target, and `./x.py test tests/mir-opt --bless` runs tests for a representative set of targets.

I find this confusing and not what I want: for one I never run tests without `--bless` because I prefer to look at a git diff. What I however like to do is pass `--keep-stage-std` when I'm working on the compiler; this doesn't work today with `--bless` because it tries a bunch of targets for which I don't have a built std.

In this PR, I make it so `./x.py test tests/mir-opt` runs the same set of tests regardless of `--bless`. By default it runs a representative sample of targets, and if `--keep-stage-std` is set it only runs tests for the current target.

src/bootstrap/src/core/build_steps/test.rs

@@ -1549,7 +1549,7 @@ impl Step for MirOpt {
             })
         };
 
-        if builder.config.cmd.bless() {
+        if builder.config.keep_stage_std.is_empty() {
             // All that we really need to do is cover all combinations of 32/64-bit and unwind/abort,
             // but while we're at it we might as well flex our cross-compilation support. This
             // selection covers all our tier 1 operating systems and architectures using only tier
@@ -1568,6 +1568,7 @@ impl Step for MirOpt {
                 run(panic_abort_target);
             }
         } else {
+            // If we're keeping a std stage, only run tests for this target.
             run(self.target);
         }
     }