ARM: Initialize LoadStore passes in TargetMachine

Initializing them in LLVMInitializeARMTarget() makes them visible early
enough for "llc -run-pass usage".

This required the pass to be renamed from "arm-load-store-opt" to
"arm-ldst-opt", because there already exists an arm-load-store-opt
cl::opt switch which would now clash with the passname getting added as
a switch in opt. On the bright side the pass name now matches the
DEBUG_TYPE name. Renamed "arm-prera-load-store-opt" to
"arm-repra-ldst-opt" as well for consistency.

llvm-svn: 275661