[BPF] Propagate NoMerge attribute when lowering function calls

`NoMerge` attribute on machine instructions prevents certain
transformations from merging these instructions.
One of such transformations is 'llvm/lib/CodeGen/BranchFolding.cpp'.

This attribute should be copied from IR `call` instructions to machine
level instructions. See `X86TargetLowering::LowerCall` as another
example.

Differential Revision: https://reviews.llvm.org/D152987