[CodeGen] Fix PrologEpilogInserter to avoid duplicate allocation of SEH structs

Summary:
When stack-protection is activated and WinEH exceptions is used, 
the EHRegNode (exception handling registration) is allocated twice on the stack.

This was not breaking anything except loosing space on the stack.

```
D:\src\llvm\examples>llc exc2.ll  -debug-only=pei
alloc FI(0) at SP[-24]
alloc FI(1) at SP[-48]   <<-- Allocated
alloc FI(1) at SP[-72]   <<-- Allocated twice!?
alloc FI(2) at SP[-76]
alloc FI(4) at SP[-80]
alloc FI(3) at SP[-84]
```

Reviewers: rnk, majnemer

Subscribers: chrisha, llvm-commits

Differential Revision: http://reviews.llvm.org/D21188

llvm-svn: 272426