[InstCombine] Replace call to haveNoCommonBitsSet in visitXor with just the...

[InstCombine] Replace call to haveNoCommonBitsSet in visitXor with just the special case that doesn't use computeKnownBits.

Summary: computeKnownBits is expensive. The cases that would be detected by the computeKnownBits portion of haveNoCommonBitsSet were already handled by the earlier call to SimplifyDemandedInstructionBits.

Reviewers: spatel, lebedev.ri

Reviewed By: lebedev.ri

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D50604

llvm-svn: 339531