Add support for dynamic stack realignment in the presence of dynamic allocas on

X86.  Basically, this is a reapplication of r158087 with a few fixes.

Specifically, (1) the stack pointer is restored from the base pointer before
popping callee-saved registers and (2) in obscure cases (see comments in patch)
we must cache the value of the original stack adjustment in the prologue and
apply it in the epilogue.

rdar://11496434

llvm-svn: 160002