[LoopUtils] Extend findStringMetadataForLoop to return the value for metadata

E.g. for:

  !1 = {"llvm.distribute", i32 1}

it now returns the MDOperand for 1.

I will use this in LoopDistribution to check the value of the metadata.

Note that the change is backward-compatible with its current use in
LoopVersioningLICM.  An Optional implicitly converts to a bool depending
whether it contains a value or not.

llvm-svn: 267190