Use "test regalloc" for the register allocator tests.

These tests were only using "test compile" because it doesn't require
any filecheck directives to be present, so just stop requiring filecheck
directives for "test regalloc" and other filecheck-based test drivers.

cranelift/filetests/regalloc/unreachable_code.cton

@@ -1,4 +1,6 @@
+; Use "test compile" here otherwise the dead blocks won't be eliminated.
 test compile
+
 set is_64bit
 isa intel haswell