Synchronize InstructionFormat and InstructionData.

These two enums must have identical variants. One is generated from the
instruction formats in meta/cretonne/formats.py, the other defines the contents
of an instruction.

Emit a conversion from InstructionData to InstructionFormat which also serves
to verify the correspondence. Rustc will error is the match is not complete.

meta/gen_instr.py

@@ -21,6 +21,20 @@ def gen_formats(fmt):
             fmt.line(f.name + ',')
     fmt.line()
 
+    # Emit a From<InstructionData> which also serves to verify that
+    # InstructionFormat and InstructionData are in sync.
+    with fmt.indented(
+            "impl<'a> From<&'a InstructionData> for InstructionFormat {", '}'):
+        with fmt.indented(
+                "fn from(inst: &'a InstructionData) -> InstructionFormat {",
+                '}'):
+            with fmt.indented('match *inst {', '}'):
+                for f in cretonne.InstructionFormat.all_formats:
+                    fmt.line(('InstructionData::{} {{ .. }} => ' +
+                              'InstructionFormat::{},')
+                             .format(f.name, f.name))
+    fmt.line()
+
 
 def collect_instr_groups(targets):
     seen = set()