[BOLT] Replace uses of layout with basic block list

As we are moving towards support for multiple fragments, loops that
iterate over all basic blocks of a function, but do not depend on the
order of basic blocks in the final layout, should iterate over binary
functions directly, rather than the layout.

Eventually, all loops using the layout list should either iterate over
the function, or be aware of multiple layouts. This patch replaces
references to binary function's block layout with the binary function
itself where only little code changes are necessary.

Reviewed By: maksfb

Differential Revision: https://reviews.llvm.org/D129585