[GlobalOpt] don't change CC of musttail calle(e|r)

When the function has musttail call - its cc is fixed to be equal to the
cc of the musttail callee. In such case (and in the case of the musttail
callee), GlobalOpt should not change the cc to fastcc as it will break
the invariant.

This fixes PR36546

Patch by: Fedor Indutny (indutny)

Differential revision: https://reviews.llvm.org/D43859

llvm-svn: 326376