StripDebugInfo: uses isa<DbgInfoIntrinsic> instead of matching against llvm.dbg.* (NFC)

Suggested by Adrian. This is NFC right now but is more clean and
robust against future potential new debug info intrinsics.

From: mehdi_amini <mehdi_amini@91177308-0d34-0410-b5e6-96231b3b80d8>
llvm-svn: 269540