[Instruction] Add updateLocationAfterHoist helper

Introduce a helper on Instruction which can be used to update the debug
location after hoisting.

Use this in GVN and LICM, where we were mistakenly introducing new line
0 locations after hoisting (the docs recommend dropping the location in
this case).

For more context, see the discussion in https://reviews.llvm.org/D60913.

Differential Revision: https://reviews.llvm.org/D85670