[TargetLowering][ARM] Don't alter opaque constants in TargetLowering::ShrinkDemandedConstant.

We don't constant fold based on demanded bits elsewhere in
SimplifyDemandedBits, so I don't think we should shrink them either.

The affected ARM test changes because a constant become non-opaque
and eventually enabled some constant folding. This no longer happens.
I checked and InstCombine is able to simplify this test. I'm not sure exactly
what it was trying to test.

Reviewed By: lebedev.ri, dmgreen

Differential Revision: https://reviews.llvm.org/D104832