Add instruction marker to MachineInstr ExtraInfo. This does almost the
same thing as Pre/PostInstrSymbols, except that it doesn't create a label until
printing instructions. This allows for labels to be put around instructions that
are deleted/duplicated somewhere.
Use this marker to track heap alloc site call instructions.
Also undo the workaround in r375137.
This is a fix for https://bugs.llvm.org/show_bug.cgi?id=43479#add_comment