[GlobalISel] Replace hard coded dynamic alloca handling with G_DYN_STACKALLOC.

This change moves the actual stack pointer manipulation into the legalizer,
available to targets via lower(). The codegen is slightly different because
we're using explicit masks instead of G_PTRMASK, and using G_SUB rather than
adding a negative amount via G_GEP.

Differential Revision: https://reviews.llvm.org/D66678

llvm-svn: 370104