X86: Avoid using _chkstk when lowering WIN_ALLOCA instructions

This patch moves the expansion of WIN_ALLOCA pseudo-instructions
into a separate pass that walks the CFG and lowers the instructions
based on a conservative estimate of the offset between the stack
pointer and the lowest accessed stack address.

The goal is to reduce binary size and run-time costs by removing
calls to _chkstk. While it doesn't fix all the code quality problems
with inalloca calls, it's an incremental improvement for PR27076.

Differential Revision: http://reviews.llvm.org/D20263

llvm-svn: 269828