MIR dumps should use "crate_name." as a prefix instead of "rustc.".

[eddyb]

rustc.main.SimplifyCfg-initial.after.mir (in src/test/mir-opt/basic_assignment) is the kind of file name we have right now, but if we used the crate name instead at the start, we'd have:
basic_assignment.main.SimplifyCfg-initial.after.mir.

Besides being more useful when dumping a bunch of stuff outside of tests, it would let us remove the directory nesting, so that file could just be in src/test/mir-opt, right next to basic_assignment.rs.

cc @rust-lang/wg-mir-opt

[eddyb]

Also, instead of 32/64 bit directories we could just add that information in the file names just before the extension, i.e. files could end in .32.diff and .{before,after}.32.mir.