[TargetLowering] SimplifyDemandedBits ZERO_EXTEND_VECTOR_INREG -> ANY_EXTEND_VECTOR_INREG

Simplify ZERO_EXTEND_VECTOR_INREG if the extended bits are not required.

Matches what we already do for ZERO_EXTEND.

Reapplies rL363850 but now with legality checks added at rL364290

llvm-svn: 364303