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/isa/intel/baseline_clz_ctz_popcount.cton

@@ -0,0 +1,104 @@
+
+test compile
+set is_64bit
+isa intel baseline
+
+
+; clz/ctz on 64 bit operands
+
+function %i64_clz(i64) -> i64 {
+ebb0(v10: i64):
+  v11 = clz v10
+  ; check: x86_bsr
+  ; check: selectif.i64
+  return v11
+}
+
+function %i64_ctz(i64) -> i64 {
+ebb1(v20: i64):
+  v21 = ctz v20
+  ; check: x86_bsf
+  ; check: selectif.i64
+  return v21
+}
+
+
+; clz/ctz on 32 bit operands
+
+function %i32_clz(i32) -> i32 {
+ebb0(v10: i32):
+  v11 = clz v10
+  ; check: x86_bsr
+  ; check: selectif.i32
+  return v11
+}
+
+function %i32_ctz(i32) -> i32 {
+ebb1(v20: i32):
+  v21 = ctz v20
+  ; check: x86_bsf
+  ; check: selectif.i32
+  return v21
+}
+
+
+; popcount on 64 bit operands
+
+function %i64_popcount(i64) -> i64 {
+ebb0(v30: i64):
+  v31 = popcnt v30;
+  ; check: iconst.i32
+  ; check: ushr
+  ; check: iconst.i64
+  ; check: band
+  ; check: isub
+  ; check: iconst.i32
+  ; check: ushr
+  ; check: band
+  ; check: isub
+  ; check: iconst.i32
+  ; check: ushr
+  ; check: band
+  ; check: isub
+  ; check: iconst.i32
+  ; check: ushr
+  ; check: iadd
+  ; check: iconst.i64
+  ; check: band
+  ; check: iconst.i64
+  ; check: imul
+  ; check: iconst.i32
+  ; check: ushr
+  return v31;
+}
+
+
+; popcount on 32 bit operands
+
+function %i32_popcount(i32) -> i32 {
+ebb0(v40: i32):
+  v41 = popcnt v40;
+  ; check: iconst.i32
+  ; check: ushr
+  ; check: iconst.i32
+  ; check: band
+  ; check: isub
+  ; check: iconst.i32
+  ; check: ushr
+  ; check: band
+  ; check: isub
+  ; check: iconst.i32
+  ; check: ushr
+  ; check: band
+  ; check: isub
+  ; check: iconst.i32
+  ; check: ushr
+  ; check: iadd
+  ; check: iconst.i32
+  ; check: band
+  ; check: iconst.i32
+  ; check: imul
+  ; check: iconst.i32
+  ; check: ushr
+  return v41;
+}