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/reader/src/parser.rs

@@ -2119,6 +2119,25 @@ impl<'a> Parser<'a> {
                 let arg = self.match_value("expected SSA value")?;
                 InstructionData::FloatCond { opcode, cond, arg }
             }
+            InstructionFormat::IntSelect => {
+                let cond = self.match_enum("expected intcc condition code")?;
+                let guard = self.match_value("expected SSA value first operand")?;
+                self.match_token(
+                    Token::Comma,
+                    "expected ',' between operands",
+                )?;
+                let v_true = self.match_value("expected SSA value second operand")?;
+                self.match_token(
+                    Token::Comma,
+                    "expected ',' between operands",
+                )?;
+                let v_false = self.match_value("expected SSA value third operand")?;
+                InstructionData::IntSelect {
+                    opcode,
+                    cond,
+                    args: [guard, v_true, v_false],
+                }
+            }
             InstructionFormat::Call => {
                 let func_ref = self.match_fn("expected function reference").and_then(
                     |num| {