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/gen_legalizer.py

@@ -355,7 +355,7 @@ def gen_xform(xform, fmt, type_sets):
 def gen_xform_group(xgrp, fmt, type_sets):
     # type: (XFormGroup, Formatter, UniqueTable) -> None
     fmt.doc_comment("Legalize the instruction pointed to by `pos`.")
-    fmt.line('#[allow(unused_variables,unused_assignments)]')
+    fmt.line('#[allow(unused_variables,unused_assignments,non_snake_case)]')
     with fmt.indented('pub fn {}('.format(xgrp.name)):
         fmt.line('inst: ir::Inst,')
         fmt.line('func: &mut ir::Function,')