llvm dev - Aug 2016 - How to modify the calling convention

I'm trying to implement something similar to this: http://gcc.gnu.org/wiki/
SplitStacks in LLVM.
What I'm hoping to do is to be able to inject the following code (in x86
asm, callee clean-up calling convention):

for each function call:
  lea %foo_return, %eax
  mov %eax, [%esp - calleestackframesize]
  push arg1
  push arg2
  push ...
  jmp foo
foo_return:
  ...

The problem I have is how to force the LLVM to generates the previous code.
How could I modify the calling sequence?

Any suggestions are appreciated.

Thanks!
Shucai
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