Fix x86 floating-point constant emission.

Previously, the basis of constant pooling was implemented, but two things were lacking:

1. The constant pools were not being emitted in the asm file.

2. A direct FP value was emitted in an FP instruction, e.g. "addss xmm0, 1.0000e00".  Curiously, at least for some FP constants, llvm-mc was accepting this syntax.

BUG= none
R=jfb@chromium.org

Review URL: https://codereview.chromium.org/291213003
diff --git a/src/IceTargetLowering.h b/src/IceTargetLowering.h
index a24c51d..92a36af 100644
--- a/src/IceTargetLowering.h
+++ b/src/IceTargetLowering.h
@@ -149,6 +149,8 @@
   virtual void addProlog(CfgNode *Node) = 0;
   virtual void addEpilog(CfgNode *Node) = 0;
 
+  virtual void emitConstants() const = 0;
+
   virtual ~TargetLowering() {}
 
 protected: