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/aliases.cton

@@ -1,4 +1,4 @@
-test compile
+test regalloc
 set is_64bit
 isa intel haswell
 

cranelift/filetests/regalloc/global-constraints.cton

@@ -1,4 +1,4 @@
-test compile
+test regalloc
 isa intel
 
 ; This test covers the troubles when values with global live ranges are defined

cranelift/filetests/regalloc/global-fixed.cton

@@ -1,4 +1,4 @@
-test compile
+test regalloc
 set is_64bit=1
 isa intel haswell
 

cranelift/filetests/regalloc/iterate.cton

@@ -1,4 +1,4 @@
-test compile
+test regalloc
 set is_64bit
 isa intel haswell
 

cranelift/filetests/regalloc/output-interference.cton

@@ -1,4 +1,4 @@
-test compile
+test regalloc
 set is_64bit=1
 isa intel haswell
 

cranelift/filetests/regalloc/schedule-moves.cton

@@ -1,4 +1,4 @@
-test compile
+test regalloc
 isa intel haswell
 
 function %pr165() native {

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
 

cranelift/src/filetest/subtest.rs

@@ -113,10 +113,5 @@ pub fn build_filechecker(context: &Context) -> Result<Checker> {
             format!("filecheck: {}", e)
         })?;
     }
-    let checker = builder.finish();
-    if checker.is_empty() {
-        Err("no filecheck directives in function".to_string())
-    } else {
-        Ok(checker)
-    }
+    Ok(builder.finish())
 }