[llvm] Address base discriminator overflow in X86DiscriminateMemOps

Summary:
Macros are expanded on a single line. In case of large expansions,
with sufficiently many instructions with memory operands (and when
-fdebug-info-for-profiling is requested), we may be unable to generate
new base discriminator values - new values overflow (base
discriminators may not be larger than 2^12).

This CL warns instead of asserting in such a case. A subsequent CL
will add APIs to check for overflow before creating new debug info.

See https://bugs.llvm.org/show_bug.cgi?id=39890

Reviewers: davidxl, wmi, gbedwell

Reviewed By: davidxl

Subscribers: aprantl, llvm-commits

Differential Revision: https://reviews.llvm.org/D55643

llvm-svn: 349075