Changes

Recommit "Add a heap alloc site marker field to the ExtraInfo in
MachineInstrs"
Summary: Fixes some things from original commit at
https://reviews.llvm.org/D69136. The main change is that the heap alloc
marker is always stored as ExtraInfo in the machine instruction instead
of in the PointerSumType because it cannot hold more than 4 pointer
types.
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.
Reviewers: rnk
Subscribers: MatzeB, hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D69536