[LiveDebugVariables] Avoid faulty addDefsFromCopies in computeIntervals

Summary:
When computeIntervals is looking through COPY instruction to
extend the location mapping for a debug variable it did not
handle subregisters correctly.

For example
    DBG_VALUE debug-use %0.sub_8bit_hi, ...
    %1:gr16 = COPY %0
was transformed into
    DBG_VALUE debug-use %0.sub_8bit_hi, ...
    %1:gr16 = COPY %0
    DBG_VALUE debug-use %1, ...
So the subregister index was missing in the added DBG_VALUE.

As long as the subreg refered to the least significant bits
of the superreg, then I guess we could get the correct
result in a debugger even when referring to the superreg.
But as in the example above when the subreg refers to other
parts of the superreg, then debuginfo would be incorrect.

I'm not sure exactly how to fix this properly, so this patch
just avoids looking through the COPY when there is a subreg
involved (for more info, see the FIXME added in the code).

Reviewers: rnk, aprantl

Reviewed By: aprantl

Subscribers: JDevlieghere, llvm-commits

Tags: #debug-info

Differential Revision: https://reviews.llvm.org/D50788

llvm-svn: 340679