[X86] Make ENDBR instruction a scheduling boundary

Instructions should not be scheduled across ENDBR instructions, as this would result in the ENDBR being displaced, breaking the parity needed for the Indirect Branch Tracking feature of CET.

Currently, the X86IndirectBranchTracking pass is later than the instruction scheduling in the pipeline, what causes the bug to be unnoticeable and very hard (if not unfeasible) to be triggered while compiling C files with the standard LLVM setup. Yet, for correctness and to prevent issues in future changes, the compiler should prevent the such scheduling.

Differential Revision: https://reviews.llvm.org/D84862