[Atomic] Remove IsStore/IsLoad in the interface, and pass the instruction instead. NFC.

Now both emitLeadingFence and emitTrailingFence take the instruction
itself, instead of taking IsLoad/IsStore pairs.
Instruction::mayReadFromMemory and Instrucion::mayWriteToMemory are used
for determining those two booleans.

The instruction argument is also useful for later D32763, in
emitTrailingFence. For emitLeadingFence, it seems to have cleaner
interface with the proposed change.

Differential Revision: https://reviews.llvm.org/D32762

llvm-svn: 302539