Add a trapff instruction.

This is the floating point equivalent of trapif: Trap when a given
condition is in the floating-point flags.

Define Intel encodings comparable to the trapif encodings.

cranelift/filetests/parser/tiny.cton

@@ -216,6 +216,9 @@ ebb0(v0: i32):
     trapz v0, stk_ovf
     v1 = ifcmp_imm v0, 5
     trapif ugt v1, oob
+    v2 = bitcast.f32 v1
+    v3 = ffcmp v2, v2
+    trapff uno v3, int_ovf
     return
 }
 ; sameln: function %cond_traps(i32)
@@ -223,5 +226,8 @@ ebb0(v0: i32):
 ; nextln:     trapz $v0, stk_ovf
 ; nextln:     $v1 = ifcmp_imm v0, 5
 ; nextln:     trapif ugt $v1, oob
+; nextln:     $v2 = bitcast.f32 $v1
+; nextln:     $v3 = ffcmp $v2, $v2
+; nextln:     trapff uno $v3, int_ovf
 ; nextln:     return
 ; nextln: }