Don't use DominatorSet::dominates for intra-block instruction dom checks.

This method is linear time in the size of the basic block, which is very
bad for large basic blocks.  On the Assembler/2004-09-29-VerifierIsReallySlow.llx
testcase, the verifier changes from taking 50s to 0.23s with this patch.

llvm-svn: 16593