[CodeGen][ObjC] Fix broken IR generated when there is a nil receiver

check

This patch fixes a bug in emitARCOperationAfterCall where it inserts the
fall-back call after a bitcast instruction and then replaces the
bitcast's operand with the result of the fall-back call. The generated
IR without this patch looks like this:

msgSend.call:                                     ; preds = %entry
  %call = call i8* bitcast (i8* (i8*, i8*, ...)* @objc_msgSend
  br label %msgSend.cont

msgSend.null-receiver:                            ; preds = %entry
  call void @llvm.objc.release(i8* %4)
  br label %msgSend.cont

msgSend.cont:
  %8 = phi i8* [ %call, %msgSend.call ], [ null, %msgSend.null-receiver ]
  %9 = bitcast i8* %10 to %0*
  %10 = call i8* @llvm.objc.retain(i8* %8)

Notice that `%9 = bitcast i8* %10` to %0* is taking operand %10 which is
defined after it.

To fix the bug, this patch modifies the insert point to point to the
bitcast instruction so that the fall-back call is inserted before the
bitcast. In addition, it teaches the function to look at phi
instructions that are generated when there is a check for a null
receiver and insert the retainRV/claimRV instruction right after the
call instead of inserting a fall-back call right after the phi
instruction.

rdar://73360225

Differential Revision: https://reviews.llvm.org/D95181