Change if() continue; to an assert if a DBG_VALUE or DBG_VALUE_LIST returns a null DILocalVariable

A  DBG_VALUE or DBG_VALUE_LIST must always return a non-null
DILocalVariable, the ARMLoadStoreOptimizer code that move’s DBG_VALUE
and DBG_VALUE_LIST instructions if their corresponding loads have been
moved, currently just continues if it finds a DBG_VALUE or
DBG_VALUE_LIST with a null DILocalVariable, change that to an assert.

Differential revision: https://reviews.llvm.org/D149762