[GlobalISel][IRTranslator] Add debug loc with line 0 to constants emitted into the entry block.

Constants, including G_GLOBAL_VALUE, are all emitted into the entry block which
lets us use the vreg def assuming it dominates all other users. However, it can
cause jumpy debug behaviour since the DebugLoc attached to these MIs are from
a user instruction that could be in a different block.

Fixes PR40887.

Differential Revision: https://reviews.llvm.org/D63286

llvm-svn: 363331