[Spectre] Fix MIR verifier errors in retpoline thunks

Summary:
The main challenge here is that X86InstrInfo::AnalyzeBranch doesn't
understand the way we're using a CALL instruction as a branch, so we
can't list the CallTarget MBB as a successor of the entry block. If we
don't list it as a successor, then the AsmPrinter doesn't print a label
for the MBB.

Fix the issue by inserting our own label at the beginning of the call
target block. We can rely on the AsmPrinter to always emit it, even
though the block appears to be unreachable, but address-taken.

Fixes PR38391.

Reviewers: thegameg, chandlerc, echristo

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D53653

llvm-svn: 345426