Simply test for available locations in optimization remarks.

When emitting optimization remarks, we test for the presence of
instruction locations by testing for a valid llvm.dbg.cu annotation.
This is slightly inefficient because we can simply ask whether the
debug location we have is known or not.

Additionally, if my current plan works, I will need to remove the
llvm.dbg.cu annotation from the IL (or prevent it from being generated)
when -Rpass is used without -g.  In those cases, we'll want to generate
line tables but we will want to prevent code generation from emitting
DWARF code for them.

Tested on x86_64.