[DebugInfo][NFC] Make is_stmt-at-block-start test X86-specific

Fixes failure on the llvm-clang-aarch64-darwin buildbot:
https://lab.llvm.org/buildbot/#/builders/190/builds/4660/

The test mentioned does not rely on any unique property of X86, but does
rely on the layout of the basic blocks produced by llc, which varies
between targets. Although the test could be duplicated for other targets,
it seems unnecessary since the behaviour being tested is not
target-specific.

Author: SLTozer