Improve and enable folding of conditional branches with tail calls.

Improve and enable folding of conditional branches with tail calls.

1. Make it so that conditional tail calls can be emitted even when
   there are multiple predecessors.

2. Don't guard the transformation behind -Os. The rationale for
   guarding it was static-prediction can be affected by whether the
   branch is forward of backward. This is no longer true for almost any
   X86 cpus (anything newer than `SnB`) so is no longer a meaningful
   concern.

Reviewed By: pengfei

Differential Revision: https://reviews.llvm.org/D140931