Add legalization patterns for fabs and fneg.

These sign bit manipulations need to use a -0.0 floating point constant
which we didn't have a way of materializing previously.

Add a ieee32.bits(0x...) syntax to the Python AST nodes that creates am
f32 immediate value with the exact requested bitwise representation.

lib/cretonne/meta/cdsl/operands.py

@@ -8,7 +8,7 @@ try:
     from typing import Union, Dict, TYPE_CHECKING, Iterable  # noqa
     OperandSpec = Union['OperandKind', ValueType, TypeVar]
     if TYPE_CHECKING:
-        from .ast import Enumerator, ConstantInt, Literal  # noqa
+        from .ast import Enumerator, ConstantInt, ConstantBits, Literal  # noqa
 except ImportError:
     pass
 
@@ -123,6 +123,15 @@ class ImmediateKind(OperandKind):
                     .format(self.name, value))
         return ConstantInt(self, value)
 
+    def bits(self, bits):
+        # type: (int) -> ConstantBits
+        """
+        Create an AST literal node for the given bitwise representation of this
+        immediate operand kind.
+        """
+        from .ast import ConstantBits  # noqa
+        return ConstantBits(self, bits)
+
     def rust_enumerator(self, value):
         # type: (str) -> str
         """