[GlobalISel] Set stack protector index when translating Intrinsic::stackprotector

Record the stack protector index in MachineFrameInfo when translating
Intrinsic::stackprotector similarly as is done by SelectionDAG when
processing the same intrinsic.

Setting this index allows the Prologue/Epilogue Insertion to recognize
that the stack protection is enabled. The pass can then make sure that
the stack protector comes before local variables on the stack and
assigns potentially vulnerable objects first so they are close to the
stack protector slot.

Differential Revision: https://reviews.llvm.org/D55418

llvm-svn: 348761