[NVPTX] Move getDivF32Level, usePrecSqrtF32, and useF32FTZ into out of...

[NVPTX] Move getDivF32Level, usePrecSqrtF32, and useF32FTZ into out of DAGToDAG and into TargetLowering.

Summary:
DADToDAG has access to TargetLowering, but not vice versa, so this is
the more general location for these functions.

NFC

Reviewers: tra

Subscribers: jholewinski, llvm-commits

Differential Revision: https://reviews.llvm.org/D28795

llvm-svn: 292693