Optimize 0.0 floating point constants. (#344)

* Optimize 0.0 floating point constants. Rather than using the existing
process of emitting bit patterns and moving them into floating point
registers, use the `xorps` instruction to zero out the register.

* is_zero predicate function will not accept negative zero. Fixed formatting for encoding recipe and filetests.

cranelift/filetests/isa/x86/optimized-zero-constants-32bit.cton

@@ -0,0 +1,19 @@
+; Check that floating-point constants equal to zero are optimized correctly.
+test binemit
+set is_64bit=0
+isa x86
+
+function %foo() -> f32 fast {
+ebb0:
+  ; asm: xorps %xmm0, %xmm0
+  [-,%xmm0]    v0 = f32const 0.0     ; bin: 0f 57 c0
+  return v0
+}
+
+function %bar() -> f64 fast {
+ebb0:
+  ; asm: xorpd %xmm0, %xmm0
+  [-,%xmm0]    v1 = f64const 0.0     ; bin: 66 0f 57 c0
+  return v1
+}
+

cranelift/filetests/isa/x86/optimized-zero-constants.cton

@@ -0,0 +1,33 @@
+; Check that floating-point constants equal to zero are optimized correctly.
+test binemit
+set is_64bit=1
+isa x86
+
+function %zero_const_32bit_no_rex() -> f32 fast {
+ebb0:
+  ; asm: xorps %xmm0, %xmm0
+  [-,%xmm0]    v0 = f32const 0.0     ; bin: 40 0f 57 c0
+  return v0
+}
+
+function %zero_const_32bit_rex() -> f32 fast {
+ebb0:
+  ; asm: xorps %xmm8, %xmm8
+  [-,%xmm8]    v1 = f32const 0.0     ; bin: 45 0f 57 c0
+  return v1
+}
+
+function %zero_const_64bit_no_rex() -> f64 fast {
+ebb0:
+  ; asm: xorpd %xmm0, %xmm0
+  [-,%xmm0]    v0 = f64const 0.0     ; bin: 66 40 0f 57 c0
+  return v0
+}
+
+function %zero_const_64bit_rex() -> f64 fast {
+ebb0:
+  ; asm: xorpd %xmm8, %xmm8
+  [-,%xmm8]    v1 = f64const 0.0     ; bin: 66 45 0f 57 c0
+  return v1
+}
+