[WebAssembly] Fix RegNumbering for the stack pointer

Previously it failed to add NumArgRegs to the offset and so clobbered an
already-used register. Now just start the numbering after the arg regs
and don't duplicate the add. Test coverage for this coming shortly with
the implementation of byval.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@258597 91177308-0d34-0410-b5e6-96231b3b80d8