ARM/MIR: Move test from MIR to CodeGen/ARM directory

test/CodeGen/MIR/ARM/ARMLoadStoreDBG.mir is an actual test for the ARM
load store optimization pass and not a test of the mir parser/printer.

It belongs to test/CodeGen/ARM; This also updates the test to use the
new -run-pass llc syntax.

llvm-svn: 275662