[WebAssembly] Emit a BasePointer when we have overly-aligned stack objects

[WebAssembly] Emit a BasePointer when we have overly-aligned stack objects

Because we shift the stack pointer by an unknown amount, we need an
additional pointer. In the case where we have variable-size objects
as well, we can't reuse the frame pointer, thus three pointers.

Patch by Jacob Gravelle

Differential Revision: https://reviews.llvm.org/D26263

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286160 91177308-0d34-0410-b5e6-96231b3b80d8