[WinEH] Make llvm.x86.seh.restoreframe work for stack realignment prologues

The incoming EBP value points to the end of a local stack allocation, so
we can use that to restore ESI, the base pointer. Once we do that, we
can use local stack allocations. If we know we need stack realignment,
spill the original frame pointer in the prologue and reload it after
restoring ESI.

llvm-svn: 241648