[tsan] Cast floating-point types correctly when instrumenting atomic accesses, compiler-rt part

Although rare, atomic accesses to floating-point types seem to be valid, i.e. `%a = load atomic float ...`. The TSan instrumentation pass however tries to emit inttoptr, which is incorrect, we should use a bitcast here. Anyway, IRBuilder already has a convenient helper function for this.

Differential Revision: https://reviews.llvm.org/D26266

llvm-svn: 286136