Rename skipDebugInfo -> skipDebugIntrinsics, NFC

This addresses post-commit feedback about the name 'skipDebugInfo' being
misleading. This name could be interpreted as meaning 'a function that
skips instructions with debug locations'.

The new name, 'skipDebugIntrinsics', makes it clear that this function
only skips debug info intrinsics.

Thanks to Adrian Prantl for pointing this out!

llvm-svn: 335667