[TargetLowering] SimplifyDemandedBits SIGN_EXTEND_VECTOR_INREG -> ANY/ZERO_EXTEND_VECTOR_INREG

Simplify SIGN_EXTEND_VECTOR_INREG if the extended bits are not required/known zero.

Matches what we already do for SIGN_EXTEND.

Reapplies rL363802 but now with legality checks added at rL364290

llvm-svn: 364299