Backport of rL326666 and rL326668 for PR36607 and PR36608.

[CallSiteSplitting] properly split musttail calls.

The original author was Fedor Indutny <fedor@indutny.com>.

`musttail` calls can't be naively splitted. The split blocks must
include not only the call instruction itself, but also (optional)
`bitcast` and `return` instructions that follow it.

Clone `bitcast` and `ret`, place them into the split blocks, and
remove the tail block when done.

Reviewers: junbuml, mcrosier, davidxl, davide, fhahn

Reviewed By: fhahn

Subscribers: JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D43729

llvm-svn: 329793