[WebAssembly] Define local sp if `llvm.stacksave` is used (#68133)

Usually `llvm.stacksave/stackrestore` are used together with `alloca`
but they can appear without it (e.g. `alloca` can be optimized away).
WebAssembly's function local physical user sp register, which is
referenced by `llvm.stacksave` is created while frame lowering and
replaced with virtual register.
However the sp register was not created when `llvm.stacksave` is used
without `alloca`, and it led MIR verification failure about
use-before-def of sp virtual register.

Resolves https://github.com/llvm/llvm-project/issues/62235