Make VerifyDomInfo and VerifyLoopInfo global variables

Verifications of dominator tree and loop info are expensive operations
so they are disabled by default. They can be enabled by command line
options -verify-dom-info and -verify-loop-info. These options however
enable checks only in files Dominators.cpp and LoopInfo.cpp. If some
transformation changes dominaror tree and/or loop info, it would be
convenient to place similar checks to the files implementing the
transformation.

This change makes corresponding flags global, so they can be used in
any file to optionally turn verification on.

llvm-svn: 292889