Introduce predicates.

Predcates are boolean functions. There will be ISA predicates and instruction
predicates.

The ISA predicates will be turned into member functions on the generated Flags
structs.

meta/isa/riscv/settings.py

@@ -3,13 +3,17 @@ RISC-V settings.
 """
 
 from cretonne import SettingGroup, BoolSetting
+from cretonne.predicates import And
+import cretonne.settings as shared
 from defs import isa
 
-isa.settings = SettingGroup('riscv')
+isa.settings = SettingGroup('riscv', parent=shared.group)
 
 supports_m = BoolSetting("CPU supports the 'M' extension (mul/div)")
 supports_a = BoolSetting("CPU supports the 'A' extension (atomics)")
 supports_f = BoolSetting("CPU supports the 'F' extension (float)")
 supports_d = BoolSetting("CPU supports the 'D' extension (double)")
 
+full_float = And(shared.enable_simd, supports_f, supports_d)
+
 isa.settings.close(globals())