[AArch64] Fix a corner case in BitFeild select

Summary:
When not useful bits, BitWidth becomes 0 and APInt will not be happy.

See https://llvm.org/bugs/show_bug.cgi?id=25571

We can just mark the operand as IMPLICIT_DEF is none bits of it is used.

Reviewers: t.p.northover, jmolloy

Subscribers: gberry, jmolloy, mgrang, aemerson, llvm-commits, rengolin

Differential Revision: http://reviews.llvm.org/D14803

llvm-svn: 254440