[FastISel][X86] Add MachineMemOperand to load/store instructions.

This commit adds MachineMemOperands to load and store instructions. This allows
the peephole optimizer to fold load instructions. Unfortunatelly the peephole
optimizer currently doesn't run at -O0.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210858 91177308-0d34-0410-b5e6-96231b3b80d8