[DAGCombine] visitMUL - allow shift by zero in MulByConstant.

This can occur under certain circumstances when undefs are created later on in the constant multipliers (e.g. in this case due to SimplifyDemandedVectorElts). Its better to let the shift by zero to occur and perform any cleanup afterward.

Fixes OSS Fuzz #15429

llvm-svn: 364179