Simply test for available locations in optimization remarks.

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211204 91177308-0d34-0410-b5e6-96231b3b80d8