[X86] Convert esp-relative movs of function arguments to pushes, step 2

This moves the transformation introduced in r223757 into a separate MI pass.
This allows it to cover many more cases (not only cases where there must be a 
reserved call frame), and perform rudimentary call folding. It still doesn't 
have a heuristic, so it is enabled only for optsize/minsize, with stack 
alignment <= 8, where it ought to be a fairly clear win.

(Re-commit of r227728)

Differential Revision: http://reviews.llvm.org/D6789


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227752 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/X86TargetMachine.cpp

@@ -193,6 +193,7 @@ public:
   void addIRPasses() override;
   bool addInstSelector() override;
   bool addILPOpts() override;
+  void addPreRegAlloc() override;
   void addPostRegAlloc() override;
   void addPreEmitPass() override;
 };
@@ -226,6 +227,10 @@ bool X86PassConfig::addILPOpts() {
   return true;
 }
 
+void X86PassConfig::addPreRegAlloc() {
+  addPass(createX86CallFrameOptimization());
+}
+
 void X86PassConfig::addPostRegAlloc() {
   addPass(createX86FloatingPointStackifierPass());
 }