[SelectionDAG] Remove ISD::FP_ROUND_INREG

I don't think anything in tree creates this node. So all of this
code appears to be dead.

Code coverage agrees
http://lab.llvm.org:8080/coverage/coverage-reports/llvm/coverage/Users/buildslave/jenkins/workspace/clang-stage2-coverage-R/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp.html

Differential Revision: https://reviews.llvm.org/D67312

llvm-svn: 371431