[MachineBasicBlock] Explicit FT branching param

Introduce a parameter in getFallThrough() to optionally
allow returning the fall through basic block in spite of
an explicit branch instruction to it. This parameter is
set to false by default.

Introduce getLogicalFallThrough() which calls
getFallThrough(false) to obtain the block while avoiding
insertion of a jump instruction to its immediate successor.

This patch also reverts the changes made by D134557 and
solves the case where a jump is inserted after another jump
(branch-relax-no-terminators.mir).

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D140790