Adds support for legalizing CLZ, CTZ and POPCOUNT on baseline x86_64 targets.

Changes:

* Adds a new generic instruction, SELECTIF, that does value selection (a la
  conditional move) similarly to existing SELECT, except that it is
  controlled by condition code input and flags-register inputs.

* Adds a new Intel x86_64 variant, 'baseline', that supports SSE2 and
  nothing else.

* Adds new Intel x86_64 instructions BSR and BSF.

* Implements generic CLZ, CTZ and POPCOUNT on x86_64 'baseline' targets
  using the new BSR, BSF and SELECTIF instructions.

* Implements SELECTIF on x86_64 targets using conditional-moves.

* new test filetests/isa/intel/baseline_clz_ctz_popcount.cton
  (for legalization)

* new test filetests/isa/intel/baseline_clz_ctz_popcount_encoding.cton
  (for encoding)

* Allow lib/cretonne/meta/gen_legalizer.py to generate non-snake-caseified
  Rust without rustc complaining.

Fixes #238.

lib/cretonne/src/ir/instructions.rs

@@ -157,6 +157,11 @@ pub enum InstructionData {
         cond: FloatCC,
         arg: Value,
     },
+    IntSelect {
+        opcode: Opcode,
+        cond: IntCC,
+        args: [Value; 3],
+    },
     Jump {
         opcode: Opcode,
         destination: Ebb,

lib/cretonne/src/verifier/mod.rs

@@ -358,6 +358,7 @@ impl<'a> Verifier<'a> {
             IntCond { .. } |
             FloatCompare { .. } |
             FloatCond { .. } |
+            IntSelect { .. } |
             Load { .. } |
             Store { .. } |
             RegMove { .. } |

lib/cretonne/src/write.rs

@@ -303,6 +303,9 @@ pub fn write_operands(
         IntCond { cond, arg, .. } => write!(w, " {} {}", cond, arg),
         FloatCompare { cond, args, .. } => write!(w, " {} {}, {}", cond, args[0], args[1]),
         FloatCond { cond, arg, .. } => write!(w, " {} {}", cond, arg),
+        IntSelect { cond, args, .. } => {
+            write!(w, " {} {}, {}, {}", cond, args[0], args[1], args[2])
+        }
         Jump {
             destination,
             ref args,