Sort allocas, compute frame pointer in Cfg pass

Split allocas in the entry block into two categories.  The first has alignment <= stack alignment and constant size.  The second violates one or both of those conditions.  Sort both of these lists in descending alignment order and emit.  Also, compute the need for a frame pointer during the pass.

BUG=
R=jpp@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1414343010 .

src/IceTargetLowering.h

diff --git a/src/IceTargetLowering.h b/src/IceTargetLowering.h
index 605562d..f518225 100644
--- a/src/IceTargetLowering.h
+++ b/src/IceTargetLowering.h
@@ -201,9 +201,11 @@
   virtual IceString getRegName(SizeT RegNum, Type Ty) const = 0;
 
   virtual bool hasFramePointer() const { return false; }
+  virtual void setHasFramePointer() = 0;
   virtual SizeT getStackReg() const = 0;
   virtual SizeT getFrameOrStackReg() const = 0;
   virtual size_t typeWidthInBytesOnStack(Type Ty) const = 0;
+  virtual uint32_t getStackAlignment() const = 0;
 
   /// Return whether a 64-bit Variable should be split into a Variable64On32.
   virtual bool shouldSplitToVariable64On32(Type Ty) const = 0;