Use DW_OP_stack_value when reconstructing variable values with arithmetic.

Use DW_OP_stack_value when reconstructing variable values with arithmetic.

When the location description of a source variable involves arithmetic
on the value itself, it needs to be marked with DW_OP_stack_value since it
is not describing the variable's location, but rather its value.

This is a follow-up to r297971 and fixes the source testcase quoted in
the comment in debuginfo-dce.ll.

rdar://problem/30725338

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301093 91177308-0d34-0410-b5e6-96231b3b80d8