[DebugInfo][CSInfo] Avoid crash when defining super-regs

In rare situations involving AVX intrinsics, it seems LLVM can be coaxed
into generating copies to arguments that look like this:

    $xmm0 = VMOVAPSrr $xmm1, implicit-def $ymm0
    CALL64 @something ymm0

This particular form of copy implicitly zeros the upper lanes of ymm0,
hence there's an implicit-def for the register in the copy. The X86
implementation of describeLoadedValue doesn't attempt to describe this sort
of copy which causes the generic implementation in
TargetInstrInfo::describeLoadedValue to fire an assertion saying it
expected the target hook to handle it.

Play it safe in the generic implementation and return the "no location /
value" return value, rather than asserting.

Differential Revision: https://reviews.llvm.org/D148626