[AArch64] Fix useful bits detection for BFM instructions

Summary:
When computing useful bits for a BFM instruction, we need
to take into consideration the case where both operands
of the BFM are equal and provide data that we need to track.

Not doing this can cause us to miss useful bits.
    
Fixes PR31138 (https://llvm.org/bugs/show_bug.cgi?id=31138)

Reviewers: t.p.northover, jmolloy

Subscribers: evandro, gberry, srhines, pirama, mcrosier, aemerson, llvm-commits, rengolin

Differential Revision: https://reviews.llvm.org/D27130

llvm-svn: 288253