[InstCombine] Don't strip function type casts from musttail calls

Summary:
The cast simplifications that instcombine does here do not make any
attempt to obey the verifier rules for musttail calls. Therefore we have
to disable them.

Reviewers: efriedma, majnemer, pcc

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D45186

llvm-svn: 329027