Remove FrameAccess struct from hasLoadFromStackSlot

This removes the FrameAccess struct that was added to the interface
in D51537, since the PseudoValue from the MachineMemoryOperand
can be safely casted to a FixedStackPseudoSourceValue.

Reviewers: MatzeB, thegameg, javed.absar

Reviewed By: thegameg

Differential Revision: https://reviews.llvm.org/D51617

llvm-svn: 341454