[WebAssembly] Put __stack_pointer in the offset field of loads and stores.

Instead of this:

i32.const       $push10=, __stack_pointer
i32.load        $push11=, 0($pop10)

Emit this:

i32.const       $push10=, 0
i32.load        $push11=, __stack_pointer($pop10)

It's not currently clear which is better, though there's a chance the second
form may be better at overall compression. We can revisit this when we have
more data; for now it makes sense to make PEI consistent with isel.

Differential Revision: http://reviews.llvm.org/D20411

llvm-svn: 270635