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/meta/base/instructions.py

@@ -485,6 +485,15 @@ select = Instruction(
         """,
         ins=(c, x, y), outs=a)
 
+cc = Operand('cc', intcc, doc='Controlling condition code')
+flags = Operand('flags', iflags, doc='The machine\'s flag register')
+
+selectif = Instruction(
+        'selectif', r"""
+        Conditional select, dependent on integer condition codes.
+        """,
+        ins=(cc, flags, x, y), outs=a)
+
 x = Operand('x', Any)
 
 copy = Instruction(