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/isa/intel/settings.py

@@ -40,6 +40,7 @@ use_lzcnt = And(has_lzcnt)
 
 # Presets corresponding to Intel CPUs.
 
+baseline = Preset(has_sse2)
 nehalem = Preset(
         has_sse2, has_sse3, has_ssse3, has_sse41, has_sse42, has_popcnt)
 haswell = Preset(nehalem, has_bmi1, has_lzcnt)