[CFLGraph] Fixed Select instruction handling

Summary:
Operand 0 is the condition, not the true value.

Use op 1 and op 2 as the correct values.

Reviewers: george.burgess.iv, nlopes, efriedma

Reviewed By: george.burgess.iv

Subscribers: craig.topper, rjmccall, lebedev.ri, llvm-commits

Differential Revision: https://reviews.llvm.org/D46343

llvm-svn: 331976