[GISel] Unify use of getStackGuard

Some rework of getStackGuard() based on comments in
https://reviews.llvm.org/D129505.

- getStackGuard() now creates and returns the destination
  register, simplifying calls
- the pointer type is passed to getStackGuard() to avoid
  recomputation
- removed PtrMemTy in emitSPDescriptorParent(), because
  this type is only used here when loading the value but
  not when storing the value

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D129576