[WebAssembly] Implement red zone for user stack

Implements a mostly-conventional redzone for the userspace
stack. Because we have unsigned load/store offsets we continue to use a
local SP subtracted from the incoming SP but do not write it back to
memory.

Differential Revision: http://reviews.llvm.org/D17525

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@261662 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed