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.

cranelift/filetests/parser/tiny.cton

@@ -42,7 +42,7 @@ ebb0:
 ; nextln:     $v3 = bxor v0, v2
 ; nextln: }
 
-; Polymorphic istruction controlled by second operand.
+; Polymorphic instruction controlled by second operand.
 function %select() {
 ebb0(v90: i32, v91: i32, v92: b1):
     v0 = select v92, v90, v91
@@ -52,6 +52,16 @@ ebb0(v90: i32, v91: i32, v92: b1):
 ; nextln:     $v0 = select $v92, $v90, $v91
 ; nextln: }
 
+; Polymorphic instruction controlled by third operand.
+function %selectif() native {
+ebb0(v95: i32, v96: i32, v97: b1):
+    v98 = selectif.i32 eq v97, v95, v96
+}
+; sameln: function %selectif() native {
+; nextln: ebb0(v0: i32, v1: i32, v2: b1):
+; nextln: v3 = selectif.i32 eq v2, v0, v1
+; nextln: }
+
 ; Lane indexes.
 function %lanes() {
 ebb0: