Expose more properties of llvm::fltSemantics

Summary: Adds accessor functions for all the fields in llvm::fltSemantics. This will be used in MergeFunctions to order two APFloats with different semanatics.

Author: jrkoenig
Reviewers: jfb
Subscribers: dschuff, llvm-commits
Differential revision: http://reviews.llvm.org/D12253

llvm-svn: 245999