[X86] Elide references to _chkstk for dynamic allocas

The _chkstk function is called by the compiler to probe the stack in an
order consistent with Windows' expectations.  However, it is possible to
elide the call to _chkstk and manually adjust the stack pointer if we
can prove that the allocation is fixed size and smaller than the probe
size.

This shrinks chrome.dll, chrome_child.dll and chrome.exe by a
cummulative ~133 KB.

Differential Revision: http://reviews.llvm.org/D17679

llvm-svn: 262370